Transmission adjustment within a wireless network for a moving vehicle

ABSTRACT

An apparatus is described, comprising circuitry to obtain base station location information for a plurality of base stations that provide a wireless network for communication with a moving vehicle, the plurality of base stations comprising a current base station and one or more other base stations, circuitry to obtain moving vehicle tracking information for the moving vehicle, circuitry to determine, based on the moving vehicle tracking information and the base station location information, transmission adjustment control information associated with each other base station, and an interface configured to transmit, for reception by the moving vehicle, the transmission adjustment control information associated with at least a selected other base station, to enable the moving vehicle to adjust a signal transmitted to the selected other base station when a handover procedure is performed to transition communication with the moving vehicle from the current base station to the selected other base station.

BACKGROUND

The present technique relates to the field of wireless communications.

It is known to provide air-to-ground (ATG) communication systems forcommunication between moving aircraft and a network of ground stations.Such systems can, for example, be used to provide a Wi-Fi hotspot withinthe aircraft in order to provide connectivity to passengers in theaircraft. With increasing demands for higher capacity, there is a desireto support modern telecommunications Standards such as 4G (LTE) in ATGsystems. However, this presents a number of technical issues.

SUMMARY

In one example arrangement, there is provided an apparatus comprising:base station location identifying circuitry to obtain base stationlocation information for a plurality of base stations that provide awireless network for communication with a moving vehicle, the pluralityof base stations comprising a current base station connected with themoving vehicle and one or more other base stations; moving vehicletracking circuitry to obtain moving vehicle tracking information for themoving vehicle; correction determination circuitry to determine, basedon the moving vehicle tracking information and the base station locationinformation, transmission adjustment control information associated witheach other base station; and an interface configured to transmit, forreception by the moving vehicle, the transmission adjustment controlinformation associated with at least a selected other base station, toenable the moving vehicle to adjust a signal transmitted to the selectedother base station when a handover procedure is performed to transitioncommunication with the moving vehicle from the current base station tothe selected other base station.

In another example arrangement, there is provided a method comprising:obtaining base station location information for a plurality of basestations that provide a wireless network for communication with a movingvehicle, the plurality of base stations comprising a current basestation connected with the moving vehicle and one or more other basestations; obtaining moving vehicle tracking information for the movingvehicle; determining, based on the moving vehicle tracking informationand the base station location information, transmission adjustmentcontrol information associated with each other base station; andtransmitting, for reception by the moving vehicle, the transmissionadjustment control information associated with at least a selected otherbase station, to enable the moving vehicle to adjust a signaltransmitted to the selected other base station when a handover operationprocedure is performed to transition communication with the movingvehicle from the current base station to the selected other basestation.

In yet another example arrangement, there is provided an apparatuscomprising: means for obtaining base station location information for aplurality of base stations that provide a wireless network forcommunication with a moving vehicle, the plurality of base stationscomprising a current base station connected with the moving vehicle andone or more other base stations; means for obtaining moving vehicletracking information for the moving vehicle; means for determining,based on the moving vehicle tracking information and the base stationlocation information, transmission adjustment control informationassociated with each other base station; and means for transmitting, forreception by the moving vehicle, the transmission adjustment controlinformation associated with at least a selected other base station, toenable the moving vehicle to adjust a signal transmitted to the selectedother base station when a handover operation procedure is performed totransition communication with the moving vehicle from the current basestation to the selected other base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technique will be described further, by way of illustrationonly, with reference to examples thereof as illustrated in theaccompanying drawings, in which:

FIG. 1 is a diagram schematically illustrating an air-to-ground (ATG)communication between an aircraft and a ground station;

FIG. 2 schematically illustrates the format of a communication frameused in one example implementation;

FIG. 3 is a block diagram illustrating components provided within avehicle terminal and a ground terminal in accordance with one examplearrangement;

FIG. 4 is a flow diagram illustrating a process performed by the vehicleterminal to determine the transmission frequency (f_(t)) of atransmitted signal;

FIG. 5 is a flow diagram showing an example of a Doppler adjustmentprocess performed by the vehicle terminal to determine the transmissionfrequency (f_(t)) of the transmitted signal;

FIG. 6 schematically shows an example of how the Doppler effect affectssignals from the ground station, and how the received signal can be usedto determine the transmission frequency (f_(t)) of the transmittedsignal;

FIG. 7 is a flow diagram showing another example of a Doppler adjustmentprocess performed by the vehicle terminal to determine the transmissionfrequency (f_(t)) of the transmitted signal;

FIGS. 8 and 9 schematically show examples of components in the vehicleterminal, used in the process of determining the transmission frequency(f_(t)) of the transmitted signal;

FIG. 10A illustrates how a connection setup signal (a RACH signal) canbe successfully communicated from a vehicle terminal to a groundterminal using the communication frame of FIG. 2 provided the vehicleterminal is no more than 108 km from the ground terminal;

FIG. 10B illustrates how when the distance between the vehicle terminaland the ground terminal exceeds 108 km the connection setup signal willnot be successfully received by the ground terminal when adopting thescheme of FIG. 10A;

FIG. 11 is a flow diagram illustrating a process performed by thevehicle terminal in accordance with one example implementation, in orderto ensure that the connection setup signal is successfully received bythe ground terminal within an identified timing window even when thedistance exceeds a setup threshold distance;

FIGS. 12A and 12B illustrate how the approach described in FIG. 11ensures correct reception of the connection setup signal, and enablesthe provision of a suitable response from the ground terminal thatallows a correct timing advance to be applied for future uplinkcommunication to the ground terminal;

FIG. 13 is a flow diagram illustrating how step 1055 of FIG. 11 may beperformed in accordance with one example implementation;

FIG. 14 is a flow diagram illustrating how step 1070 of FIG. 11 may beperformed in one example implementation;

FIG. 15 is a diagram schematically illustrating a scheduling issue thatcan arise when the vehicle terminal is separated from the groundterminal by a distance exceeding a scheduling threshold distance;

FIGS. 16A and 16B are a flow diagram illustrating a process performed bythe ground terminal in order to resolve the scheduling issue illustratedin FIG. 15, in accordance with one example arrangement;

FIGS. 17A to 17C illustrate how the process of FIGS. 16A and 16B may beapplied for various separation distances between the vehicle terminaland the ground terminal, in accordance with one example arrangement;

FIG. 18 illustrates multiple communication frame formats that can besupported in one example implementation;

FIG. 19 is a flow diagram illustrating how the ground terminal in oneexample implementation can switch between the communication frameformats of FIG. 18 as separation distances permit, in order to seek toincrease the proportion of the communication frame available fordownlink communications;

FIG. 20 is a diagram schematically illustrating air-to-ground (ATG)communication between an aircraft and a ground station, and a selectedother ground station for communication to be transitioned to during ahandover procedure;

FIG. 21 is a block diagram schematically illustrating components of anair-to-ground (ATG) communication network;

FIG. 22 is a flow diagram illustrating an example of a method ofdetermining and transmitting transmission adjustment controlinformation;

FIG. 23 is a flow diagram illustrating a process for determining andtransmitting transmission adjustment control information for a networkof base stations;

FIGS. 24 to 27 are flow diagrams illustrating processes for determiningfrequency correction information for a given base station;

FIG. 28 is a timing diagram illustrating signals passed between an airterminal (AST), its connected base station (GBS) and an air-to-groundmanager (ATGM);

FIG. 29 is a diagram schematically illustrating the fields of an IPpacket used to transmit signals within an air-to-ground (ATG)communication network.

DESCRIPTION OF EXAMPLES

As mentioned earlier, a number of technical issues can arise whenseeking to support modern telecommunications Standards such as 4G (LTE)in systems such as ATG systems. One particular issue that arises isinterference between carrier signals due to the impact of the Dopplereffect on the frequencies of signals transmitted between the groundterminal and the vehicle terminal (in the aircraft). This isparticularly significant in modern telecommunications Standards such as4G, due to the high frequency of signals that are transmitted accordingto these Standards—coupled with the high speeds with which modernaeroplanes travel, this means that the Doppler effect can be significantin ATG communication, since the Doppler effect is dependent on both thevelocity of the vehicle and the frequency of the signal.

While it may be possible to mitigate some of the problems caused by theDoppler effect by choosing modulation schemes for the signals that aremore resilient to interference, such schemes typically result in reducedthroughput, which has the unwanted effect of lowering the capacity ofcommunication in the system. The present technique, therefore, aims toovercome some of the issues related to the Doppler effect in ATGcommunication, without significantly reducing the capacity.

An additional technical issue that can arise is in relation to a timingdelay of a transmitted signal, due to the separation distance betweenthe transmitter and the receiver. This issue is of particular concernduring a sign-on procedure to seek to establish a communication linkbetween the vehicle terminal (air terminal) and the ground terminal(base station)—for example, when using a modern telecommunicationsStandard such as 4G (LTE), it is necessary during the sign-on procedurefor the vehicle terminal to issue a connection setup signal so that itcan be received by the ground terminal within an identified timingwindow. There are various formats of connection setup signal that can beused, but the maximum separation distance between the moving vehicle andground terminal that can be supported in 4G (LTE) is of the order ofapproximately 100 km. If the separation distance exceeds that, then theconnection setup signal will not be received within the specified timingwindow, and a communication link will hence not be established. However,in known ATG systems, the network of ground terminals may be such thatthe separation distance between the aircraft and the ground terminalwith which a communication link is sought to be established may be up to400 km.

Whilst an aircraft is given as an example of a moving vehicle to whichthe techniques described herein may be applied, the techniques can beapplied to other types of moving vehicles, for example a train, wherethe ground terminals may typically be spread out along the track.

The techniques described herein recognise that a mechanism is needed bywhich timing and frequency corrections for signals to be transmitted bythe vehicle terminal can be determined. Moreover, the inventorsrecognised that it would also be beneficial to determine frequency andtiming corrections for other base stations not currently incommunication with the moving vehicle, for example to ensure that ahandover procedure to transition communication with the moving vehiclefrom the current base station to the selected other base station can becarried out more smoothly.

In one example arrangement, the present technique provides an apparatusincluding base station location identifying circuitry to obtain basestation location information for a plurality of base stations thatprovide a wireless network for communication with a moving vehicle, theplurality of base stations comprising a current base station connectedwith the moving vehicle and one or more other base stations. Theapparatus also comprises moving vehicle tracking circuitry to obtainmoving vehicle tracking information for the moving vehicle andcorrection determination circuitry to determine, based on the movingvehicle tracking information and the base station location information,transmission adjustment control information associated with each otherbase station. An interface of the apparatus is then configured totransmit, for reception by the moving vehicle, the transmissionadjustment control information associated with at least a selected otherbase station, to enable the moving vehicle to adjust a signaltransmitted to the selected other base station when a handover procedureis performed to transition communication with the moving vehicle fromthe current base station to the selected other base station.

According to the present technique, as described above, transmissionadjustment control information may be determined for a plurality of basestations in a neighbourhood of candidate base stations for connectionwith the vehicle terminal. These corrections may be transmitted forreception by the moving vehicle, allowing the capabilities of 4G (LTE)communication to be extended to larger ranges and higher speeds ofmoving vehicle. The determination of the transmission adjustment controlinformation is performed centrally (e.g. in a separate apparatus withinthe network, rather than at the vehicle terminal). This allowscorrections to be determined for base stations which are not yetconnected with the moving vehicle, allowing handover to these other basestation to be performed more smoothly. Moreover, determining thetransmission adjustment control information centrally, rather than ineither the base station or the air terminal, allows the presenttechnique to be more easily integrated into existing communicationsnetworks, operating according to existing Standards such as 4G (LTE).

In some examples, the transmission adjustment control informationcomprises at least one of frequency adjustment information and timingadjustment information. The frequency adjustment information isindicative of a frequency adjustment to be applied to a transmissionfrequency of a signal transmitted by the moving vehicle, so as to reducea frequency difference between an observed frequency of that signal atthe selected base station and a predetermined uplink frequency, and thetiming adjustment information indicative of a timing adjustment to beapplied to a transmission time of such a signal, so as to reduce atiming difference between a reception timing of that signal at theselected base station and an expected timing.

The predetermined uplink frequency is a frequency at which the selectedbase station expects to receive an uplink (also referred to as reverselink herein) signal from the moving vehicle, whereas the observedfrequency is the actual frequency of that signal. Regarding the timingadjustment information, the reception timing refers to a time or a timeslot in which the uplink signal is received at the selected basestation, whereas the expected timing refers to a time or time slot (forexample, a particular sub-frame or group of sub-frames) within which thebase station expects to receive the signal. An uplink signal is a signaltransmitted from the moving vehicle to a base station, whereas adownlink (also referred to as forward link herein) signal is a signaltransmitted from a base station to the moving vehicle.

According to the above example, a Doppler correction can be applied tothe frequency of signals transmitted by the moving vehicle, or a timingadjustment can be applied to the transmission timing of a signal. Forexample, the timing correction could be applied during a sign-onprocedure between the moving vehicle and the selected other basestation, in accordance with a handover procedure. The apparatus iscapable of transmitting the timing adjustment information, the frequencyadjustment information, or both. It should be noted that communicationwithin systems such as 4G (LTE) is carried out using communicationframes, and the reception timing of the signal received by the selectedbase station may be specified with reference to a particular feature ofthe communication frame, such as the start of the frame. Moreover, aswill be discussed in more detail below, the reception timing may referto a particular sub-frame within the communication frame.

In some examples, the adjustment control information comprises absoluteadjustment control information or relative adjustment controlinformation. The absolute adjustment control information comprises atleast one of an absolute frequency adjustment and an absolute timingadjustment to be applied to the signal as generated by a terminal deviceof the moving vehicle, and the relative adjustment control informationcomprises at least one of a relative frequency adjustment and a relativetiming adjustment to be applied to the signal in addition to at leastone of an existing frequency adjustment and an existing timingadjustment.

Thus, the techniques described herein can be applied either in additionto or in place of an alternative mechanism for adjusting the frequencyor the timing of signals transmitted by the moving vehicle. For example,the techniques can be applied regardless of whether the moving vehicleitself is also capable of performing timing and frequency adjustments.Thus, the system is versatile and can be applied regardless of theexistence or otherwise of additional mechanisms.

In some examples, the moving vehicle tracking information comprisesinformation indicative of a location and a velocity of the movingvehicle.

The Doppler effect on the frequency of signals transmitted by the movingvehicle depends on the velocity of the vehicle, and also depends on theseparation distance between the vehicle and the base station. Similarly,the timing offset is also dependent on the separation distance. From theposition information of the base station (identified by the base stationlocation identifying circuitry) and of the moving vehicle (part of themoving vehicle tracking information), the vector or scalar separationdistance between the moving vehicle and the base station can becalculated, enabling an accurate determination of the transmissionadjustment control information to be made (with the velocity also beingneeded in the case where the transmission adjustment control informationis frequency adjustment information).

There are a number of ways in which the moving vehicle trackingcircuitry may determine the location and velocity of the moving vehicle.In some examples, the interface is configured to receive, from thecurrent base station, identification information of the moving vehicle,and the moving vehicle tracking circuitry is configured to obtain thelocation and the velocity of the moving vehicle by accessing a trackinginformation database using the identification information of the movingvehicle.

Thus, the present technique can make use of existing,publicly-accessible, tracking information databases (such as aviationdatabases, in the case where the moving vehicle is an aircraft). Thisensures that the moving vehicle tracking circuitry is able to determineup-to-date tracking information for the vehicle, without requiring suchinformation to be transmitted by the vehicle. However, in an alternativeimplementation such information could be transmitted by the vehicleitself.

As mentioned above, both the timing delay and the Doppler effect dependon the separation distance between the moving aircraft and the basestation. Thus, in order to calculate the correction information ineither case, some examples of the apparatus comprise distancecomputation circuitry configured to determine, for each other basestation, separation information indicating a separation between themoving vehicle and that other base station based on the location of themoving vehicle and a location of that other base station.

Thus, based on the locations of the base station and the vehicle, theapparatus can calculate the separation distance, in order to accuratelydetermine the transmission adjustment control information.

In particular, the Doppler effect relies on the vector separationbetween the vehicle and the base station. Thus, in some examples, wherethe transmission adjustment control information comprises said frequencyadjustment information, the separation information identifies a vectorseparation. The correction determination circuitry is then configured todetermine the frequency adjustment information associated with eachother base station based on the velocity of the moving vehicle and thevector separation between the moving vehicle and that other basestation.

Such an approach can enable an accurate determination of the frequencycorrection information.

On the other hand, when the transmission adjustment control informationcomprises said timing adjustment information, the correctiondetermination circuitry may be configured to determine the timingadjustment information associated with each other base station based onthe separation (which can be presented as a scalar value) between themoving vehicle and that other base station.

This ensures the accuracy of the determination of the timing correctioninformation.

The one or more other base stations for which the transmissionadjustment control information is calculated may form part of aneighbourhood of base stations in range of the moving vehicle—the otherbase stations are thus candidate base stations for a handover procedure.The other base stations may be identified in a number of ways; in someexamples, the base station location identifying circuitry is configuredto identify the one or more other base stations with reference to abearing of the moving vehicle.

The bearing refers to the direction a vehicle is facing, which may bedifferent to the direction in which it is travelling. For example, ifthe vehicle is an aircraft, the bearing of the vehicle refers to thedirection in which the nose of the aircraft is pointing, and may differfrom the direction of travel due to factors such as wind. The aboveexample enables the potential pool of potential handover candidates (forexample as determined from the network neighbourhood information for thecurrently connected base station) to be narrowed down in accordance withfactors which may affect the signal quality, should a particular basestation be selected as a candidate for a handover procedure. It isparticularly useful to consider the bearing of the vehicle whenselecting the other base stations, since the signal strength between agiven base station and the vehicle may depend on the directionality oftransmitted signals. For example, the vehicle may have an antenna in aparticular location (e.g. on one side), which enables a stronger signalstrength for base stations closer to that part of the vehicle. Thus, thebearing (or heading) of the vehicle will affect the signal strength. Byselecting the other base stations in dependence on the bearing, thenumber of base stations for which the calculation is made can benarrowed down, avoiding wasting time or power on calculating correctioninformation for base stations which are unlikely to be selected ascandidates for the handover procedure. For example, avoiding wastingtime can allow for a handover between base stations to take place morequickly, reducing any interruption to traffic flows that could occurwhen a handover is not carried out in a timely manner.

Furthermore, the effect of bearing on the signal strength isparticularly pronounced in ATG communication, since aeroplanes are oftenlarge, and hence for example the fuselage of the aircraft may partiallyobscure the aircraft's antenna(s) from one or more base stations.Furthermore, as mentioned above, the difference between the bearing andthe direction of travel may be more noticeable for aircraft than forother vehicles, since they are more strongly affected by the wind thanground-based vehicles. Thus, using the bearing in the identification ofthe one or more other base stations is particularly beneficial in ATGcommunication networks. It is noted, however, that this feature mayapply to any moving vehicle, even in the case where the bearing and thedirection of travel are the same.

While it is particularly beneficial to calculate transmission adjustmentcontrol information for a plurality of base stations not currently incommunication with the moving vehicle—as discussed in the aboveexamples—it is also beneficial to perform the same determination for thecurrent base station. Thus, in some examples, the correctiondetermination circuitry is configured to perform a process ofdetermining further transmission adjustment control informationassociated with the current base station, and the interface isconfigured to transmit, for reception by the moving vehicle, the furthertransmission adjustment control information, to enable the movingvehicle to adjust at least one further signal transmitted to the currentbase station.

In this way, ongoing adjustments can be made to the frequency and timingof signals transmitted by the moving vehicle to the current basestation.

In some examples, the correction determination circuitry is configuredto iteratively perform the above process of determining furthertransmission adjustment control information, to enable ongoingadjustment of signals to be transmitted by the moving vehicle to thecurrent base station.

In this way, continuous corrections to signals transmitted from themoving vehicle to the current base station can be made, based on thetransmission adjustment control information. This can allow high signalquality with few interruptions to be maintained throughout the period inwhich the vehicle is in communication with the current base station.

When determining the further transmission adjustment control informationfor the current base station, a number of different techniques can beemployed. In some examples, the interface is configured to receiveoffset information for a plurality of previous signals received at thecurrent base station from the moving vehicle, the offset informationcomprising at least one of frequency offset information indicative of adifference between an observed frequency of each of the plurality ofprevious signals received at the current base station and apredetermined uplink frequency of that previous signal, and timingoffset information indicative of a difference between a reception timingof each of the plurality of previous signals at the current base stationand an expected timing for that previous signal. The correctiondetermination circuitry is then configured to determine the furthertransmission adjustment control information based on the offsetinformation.

In this way, the correction determination circuitry does not need toknow the vehicle tracking information or the location of the currentbase station in order to make an accurate determination of the furthertransmission adjustment control information. Instead, the determinationis made based on previous signals transmitted by the moving vehicle tothe base station.

This can be done in a number of ways. In some examples, the correctiondetermination circuitry may be configured to determine the furthertransmission adjustment control information by calculating a filteredestimate of the ongoing adjustments to be made to transmitted signals,from the offset information received for said plurality of previoussignals. For example, a Kalman filter can be used to estimate theongoing adjustments, using a series of measurements observed over time.Alternatively, another type of filtering mechanism could be used.

However, the above mechanism for determining the further transmissionadjustment control information is not the only mechanism that can beemployed. In some examples, where the further transmission adjustmentcontrol information associated with the current base station comprisesfrequency adjustment information associated with the current basestation, the distance computation circuitry is configured to determine,for the current base station, a vector separation between the movingvehicle and the current base station based on the location of the movingvehicle and a location of the current base station. The correctiondetermination circuitry is then configured to determine the frequencyadjustment information associated with the current base station based onthe vector separation between the moving vehicle and the current basestation and the velocity of the moving vehicle.

In some examples, where the further transmission adjustment controlinformation associated with the current base station comprises timingadjustment information associated with the current base station, thedistance computation circuitry may be configured to determine, for thecurrent base station, further separation information indicating aseparation between the moving vehicle and the current base station basedon the location of the moving vehicle and a location of the current basestation. The correction determination circuitry is then configured todetermine the timing adjustment information associated with the currentbase station based on the separation between the moving vehicle and thecurrent base station.

In this way, the frequency adjustment information and the timingadjustment information associated with the current base station can eachbe calculated even if no offset information has been received. Thisimproves the versatility of the system.

As mentioned above, the interface is configured to transmit thetransmission adjustment control information so that it can be receivedby the moving vehicle. This can be achieved in a variety of ways. Insome examples, the interface is configured to transmit the transmissionadjustment control information to the current base station, forreception by the moving vehicle. The current base station can thenpropagate the information on to the moving vehicle via the wirelesscommunication link established between it and the moving vehicle.

In existing 4G (LTE) systems, communication with an item of userequipment is typically effected via the current base station. Thus, theabove feature allows the system to be integrated with existing 4Gsystems, improving the compatibility of this new system with existingsystems.

In some examples, the transmission adjustment control information istransmitted in an IP (Internet Protocol) packet comprisingidentification information of one of the plurality of base stations, arelative bit indicative of whether the transmission adjustment controlinformation comprises relative adjustment control information orabsolute adjustment control information, the transmission adjustmentcontrol information, and identification information of the movingvehicle.

In this way, all of the relevant information is transmitted to thecurrent base station in a form that is compatible with the 4G Standard,further improving the compatibility of the system with existing systems.

As discussed above, the present technique may be particularly usefulwhen preparing to carry out a handover procedure, to transfer a movingvehicle from the current base station to a selected other base station.Thus, in some examples, the interface is also configured to transmit thetransmission adjustment control information associated with the selectedother base station for reception by said selected other base station.

This might involve the apparatus transmitting the transmissionadjustment control information associated with the selected other basestation directly to that base station, or it might send the informationto the current base station to be forwarded to the selected basestation. In this way, the selected other base station can be providedwith information about the adjustments to the frequency and timing ofsignals that will be transmitted to it by the moving vehicle, which canenable a smoother handover.

In particular, in some examples, the transmission adjustment controlinformation associated with the selected other base station comprisestiming adjustment information associated with the selected other basestation, and this timing adjustment information can be used by theselected other base station to determine a reception timing of saidsignal transmitted to the selected other base station when said handoverprocedure is performed to transition communication with the movingvehicle from the current base station to the selected other basestation.

In this way, the selected other base station may be advised upfront ofwhich sub-frame or sub-frames in which to expect reception of a sign-onsignal during the handover procedure, reducing the monitoring burden onthe selected other base station, and hence improving the efficiency ofthe handover procedure.

As mentioned throughout the above examples, the above techniques can beapplied to any of a number of different types of moving vehicle.However, in some examples, the moving vehicle is an aircraft.

As noted above, the high speed of aircraft and the large separationdistance between base stations in ATG networks mean that the abovetechniques are particularly useful for communication with aircraft.

Particular examples will now be described with reference to the Figures.

The moving vehicles for which the techniques described herein can beutilised can take a variety of forms. For instance, the techniques couldbe applied in respect of trains, where the ground terminals may bespread out along the track. However, for the purposes of the examplesdiscussed herein, it will be assumed that the moving vehicle is anaircraft, such as the airplane 10 shown in FIG. 1. As shown in FIG. 1,the airplane 10 is able to communicate with a ground terminal 20 (whichmay also be referred to herein as a ground station). A network of groundterminals will be provided, enabling the aircraft 10 to connect todifferent ground terminals during a flight in order to seek to maintaina communication link that can be used to provide connectivity topassengers in the aircraft. As shown in FIG. 1, the aircraft 10 isassumed to be travelling at a velocity 40, and has a relative separation30 between it and the ground terminal that it is connected to. Thisrelative separation can be specified as a vector, as can the velocity40, and there will be an angular separation between the velocity vectorand the relative separation vector, namely the angle 50 shown in FIG. 1.

Communication between the aircraft 10 and a ground station 20 with whicha communication link is established can take place within communicationframes. An example communication frame that may be used is illustratedin FIG. 2. Here, the communication frame 60 is defined in both thefrequency and time domains. In particular, in the time domain, the framecan be considered as consisting of a plurality of sub-frames 70. In oneparticular example, a communication frame 60 is 10 milliseconds (ms)long, and there are ten sub-frames in the communication frame, whereeach sub-frame has a duration of 1 ms. Each sub-frame 70 comprises anumber of resource blocks (the resource blocks not being shownseparately in FIG. 2), a resource block being the smallest allocableportion of the communication frame.

A sub-frame may be allocated for downlink communication (also referredto herein as forward link communication) from a ground terminal 20 tothe aircraft 10, or can be allocated for uplink communication (alsoreferred to herein as reverse link communication) from the aircraft 10to the ground terminal 20. In FIG. 2, sub-frames allocated for downlinkcommunication are prefixed with the letter “D” and sub-frames allocatedfor uplink communication are prefixed with the letter “U”. As also shownin FIG. 2, one or more sub-frames may be allocated as special sub-frames(prefixed by the letter “S”). These can be used as a gap sub-frame toprovide some separation between downlink communication and uplinkcommunication. However, it is possible that not the entirety of thespecial sub-frame is left as a gap. In particular, each sub-frame can beconsidered as consisting of a plurality of symbols, in one particularexample there being 14 symbols within each sub-frame. Hence, one or moreof the symbols may be allocated for downlink communication and one ormore of the symbols may be allocated for uplink communication, with theremaining symbols being left free. In one specific implementation of thecommunication frame format shown in FIG. 2, the first three symbolswithin the special sub-frame S0 can be used for downlink communication,and the final symbol may be used for uplink communication. This leaves10 symbols free, which in one embodiment equates to a 0.712 ms gap.

FIG. 3 is a block diagram illustrating more details of the componentsprovided within a vehicle terminal 100 and a ground terminal 130. Thevehicle terminal 100 may for example be provided within the aircraft 10shown in FIG. 1, whereas the ground terminal 130 may form the groundstation 20 shown in FIG. 1.

The vehicle terminal 100 has an antenna system 105 used to communicatewirelessly with the ground terminal 130. The antenna system 105 mayinclude all of the electronics used to convert between baseband and RFsignals for both data to be transmitted from the vehicle terminal'santenna and for data received by the vehicle terminal's antenna.Communication control circuitry 110 is provided for controlling theoperation of the antenna system 105. To assist the communication controlcircuitry 110 in performing the control operations to be described inmore detail herein, the communication control circuitry 110 has accessto distance computation circuitry 120 that can be used to determine theseparation between the vehicle terminal 100 and the ground terminal 130.In some example implementations, that separation is expressed as avector identifying the relative separation between the two antennasystems, whilst in other implementations that separation may beexpressed as an absolute separation distance (i.e. a scalar term ratherthan a vector term).

The distance computation circuitry 120 may have access to locationspecifying circuitry 115 that can provide information identifying thecurrent location of the vehicle terminal 100. The location specifyingcircuitry can take a variety of forms, but in one example implementationis a GPS receiver.

The distance computation circuitry 120 can be arranged to operate in avariety of ways, but in one example implementation extracts informationfrom a downlink communication in order to seek to identify the locationof the ground terminal 130. That information could in principle directlyidentify the coordinates of the ground terminal, but in one exampleimplementation that information is an identifier of the ground terminal,and the distance computation circuitry uses that identifier in order toobtain the coordinates of the ground terminal.

In particular, as shown in FIG. 3, in one example implementation thevehicle terminal 100 has a storage device 125 providing a correlationbetween ground terminal identifiers and associated location information.Accordingly, a lookup operation can be performed within the storageusing the identifier information extracted from the downlink signal, inorder to obtain the location information of the ground terminal. Usingthat information, and the location information from the GPS receiver115, the distance computation circuitry 120 can then calculate theseparation between the vehicle terminal and the ground terminal.

As shown in FIG. 3, the ground terminal will include a further antennasystem 135, which is controlled by communication link establishing andscheduling circuitry 140. The functionality performed by thecommunication link establishing and scheduling circuitry 140 will bediscussed in more detail later. However, in one implementation thatcomponent has access to distance computation circuitry 145 that cancompute the separation between the ground terminal 130 and the vehicleterminal 100. As with the earlier-discussed distance computationcircuitry 120, the distance computation circuitry may produce thatseparation as a vector quantity, or as a scalar quantity dependent onimplementation. In one example implementation, the distance computationcircuitry will know the coordinate information of the ground terminal130, which it will be appreciated is fixed, and will obtain vehicletracking information indicative of the current location of the vehicleterminal 100. This vehicle tracking information can be obtained in avariety of ways. However, considering the example of an aircraft 10shown in FIG. 1, it will be appreciated that there are availableresources that track in real time the coordinates of aircrafts, and thatinformation can be obtained in order to provide the distance computationcircuitry 145 with the required vehicle tracking information for thevehicle terminal 100.

The separation between the ground terminal 130 and the vehicle terminal100 determined by the distance computation circuitry 120 is calculatedas a vector value, indicating both a magnitude (distance) and direction(angle). In one example implementation, analysis circuitry performs aDoppler adjustment process to determine an adjustment to be made to thetransmission frequency of the uplink (reverse link) signal, based on thevector separation determined by the distance computation circuitry. Theanalysis circuitry therefore encompasses the distance computationcircuitry 120 and at least some of the functionality of thecommunication control circuitry 110. The transmitted frequency (f_(t))of the transmitted signal (uplink signal) is determined such that theobserved frequency of the uplink signal when it is received by thefurther antenna system 135 equals a predetermined uplink frequency(f_(UL)); this is the frequency at which the ground terminal 130 expectsto receive the uplink signal, corrected (by the Doppler adjustmentprocess) to account for the Doppler effect in both the received andtransmitted signals. The Doppler adjustment process is described in moredetail with reference to the examples given below.

FIG. 4 is a flow diagram illustrating a method of operation of thevehicle terminal 100. In a first step S402, a received signal (thedownlink/forward link signal) is received at the antenna of the antennasystem 105, the received signal having a received frequency (f_(r)). Atleast one item of information—for example, information with which thedistance computation circuitry 120 can determine the vector separationbetween the antenna system 105 of the vehicle terminal 100 and thefurther antenna system 135 of the ground terminal 130—is obtained atstep S404 from the received signal by the distance computation circuitry120. The information is then used in a Doppler adjustment process S406,to determine the transmitted frequency (f_(t)) with which the uplinksignal is to be transmitted, taking into account any frequency shiftsdue to the Doppler effect.

Once the Doppler adjustment process S406 has been performed, then atstep S408 the antenna system 105 can transmit, at the transmittedfrequency (f_(t)), the uplink signal to the further antenna system 135.

FIG. 5 is a flow diagram showing an example of the Doppler adjustmentprocess S406 a referred to in FIG. 4. This particular example refers tothe case where the information extracted from the received signal is anidentifier of the ground terminal 130.

The Doppler adjustment process of this example starts at a step S502. Atstep S504 the distance computation circuitry 120 obtains, from thereceived downlink signal, an identifier of the ground terminal 135.Using this identifier, the computation circuitry 120 can then index thestorage structure 125 in order to determine at step S506 the location ofthe ground station. The location of the vehicle terminal, along with itsvelocity, are also determined at step S508. At least the location can bedetermined by the location specifying circuitry 115, but in instanceswhere the location specifying circuitry 115 is a GPS receiver it will beappreciated that the velocity information can also be determined fromthe output of the GPS receiver. Using the locations of the groundterminal 130 and the vehicle terminal 100, the vector displacement(separation) between the two terminals can be determined at step S510 bythe distance computation circuitry 120, and thus an adjustment value(Δf) representative of the change in frequency of the received signaldue to the Doppler effect can be calculated at step S512. Thiscalculation is performed by the analysis circuitry according to theDoppler formula:

${\Delta \; f} = {\frac{r \cdot v}{{r}c}f_{DL}}$

where r is the vector separation between the ground terminal 130 and thevehicle terminal 100, v is the velocity of the vehicle terminal 100, cis the speed of light and f_(DL) is the predetermined downlink frequency(the frequency at which the ground terminal 130 transmits the downlinksignal).

This adjustment value (Δf) is then used to calculate the transmittedfrequency (f_(t)) with which the uplink signal is to be transmitted, inaccordance with the following formula:

f _(t) =f _(r)−2Δf

where f_(r) is the received frequency of the downlink signal. The abovecalculations assume that a time division duplex (TDD) scheme isemployed, in which the predetermined uplink frequency and thepredetermined downlink frequency (the frequencies of the uplink anddownlink signals at the ground terminal) are the same. The receivedfrequency of the downlink signal is f_(r)=f_(DL)+Δf, and that receivedfrequency is used as the default frequency for transmission from thevehicle terminal 100. Hence the frequency of the transmitted signalneeds to be adjusted by −2Δf in order to compensate for the Dopplereffect in both the received and transmitted signals, such that thefrequency of the uplink signal as observed by the ground terminal isf_(UL)=f_(DL).

However, the above approach can also be generalised to a frequencydivision duplex (FDD) scheme where the predetermined uplink and downlinkfrequencies differ, as discussed below with reference to FIG. 9, and theadjustment required to the default transmission frequency in that caseis the same.

While the example described with reference to FIG. 5 assumes that anidentifier of the ground station 130 is obtained from the downlinksignal, it is also possible for the downlink signal itself to specifythe location (e.g. the coordinates) of the ground terminal 130. In thiscase, steps S504 and S506 in FIG. 5 would be replaced with a single stepof obtaining, from the received signal, the location of the groundterminal 130.

Furthermore, in some examples it may also be possible to calculate theDoppler adjustment Δf without knowing the magnitude of the distance (r)between the two terminals, provided that at least the angle θ betweenthe vehicle's velocity and a line connecting the two terminals is known.This is because the dot product between r and v can be calculated as|r|*|v|*cosθ, so that |r| cancels out in the Doppler formula. The angleθ could be calculated in any of a number of ways; for example, the angleof arrival (AoA) of the incoming downlink signal could be determinedusing a phase array, to determine the angle relative to the vehicle'sheading.

The examples described so far involve calculating, with distancecomputation circuitry 120, the vector separation between the groundterminal 130 and the vehicle terminal 100. However, other examplesinstead perform the Doppler adjustment process using information aboutthe received signal itself, rather than information about the groundterminal 130 (such as its location). One such example is demonstratedschematically in FIG. 6. A ground station 130 and an air station 600 (anexample of the vehicle terminal 100 shown in FIG. 3) are shown in FIG.6. The ground station 130 transmits a downlink signal 602 at a frequency(the predetermined downlink frequency f_(DL)) of 2.4 GHz (2.4 billioncycles per second). This signal is received a short time later at theair station 600, which is moving away from the ground station 130 with agiven velocity (v). Due to the Doppler effect, the frequency of thesignal as observed by the air station 600 is lower than 2.4 GHz (orhigher if the air station 600 is moving towards the ground station 100),meaning that the number of cycles per second has reduced. In thisexample, the frequency (f_(r)) of the downlink signal 602 as observed bythe air station 600 can be compared with the expected value of thefrequency (2.4 GHz) to determine an adjustment (Δf) to be applied to thetransmitted signal (not shown).

The air station 600 also receives a timing signal 604 from a GPSsatellite 606, which provides accurate timing information. This timinginformation can then be used by the air station 600 (more particularly,by the analysis circuitry in the air station 600) to accurately countthe number of cycles per second in the received signal 602, to determinehow the frequency has changed. This information can then be used by theanalysis circuitry to determine the transmitted frequency (f_(t)) of thetransmitted signal. Thus, FIG. 6 is an example of the use of informationrelating to the received signal itself in performing a Doppleradjustment process.

While the arrangement shown in FIG. 6 calculates the received frequency(f_(r)) of the downlink signal as the information relating to thereceived signal, there are other examples of information about thereceived signal that could be used instead, for example the number ofcommunication frames 60 received at the air station 600 per second(which can be compared to the expected value of 100 per second), thenumber of OFDM (Orthogonal Frequency Division Multiplexing) symbolsreceived per second, or the number of primary synchronisation signals(PSSs) counted per second. In fact, any property of the received signalthat is affected by the Doppler effect (so any property related to thefrequency of the signal) can be used.

FIG. 7 is a flow diagram illustrating another example of the Doppleradjustment process S406 b applied in FIG. 4, this time using informationrelating to the received signal, rather than information about theground terminal 130. In the following example, it is assumed that a TDDscheme is employed, and that the predetermined uplink frequency andpredetermined downlink frequency are, therefore, the same.

In FIG. 7, the process begins at a first step S702, before passing to astep S704 of obtaining, from the received signal, information relatingto the received signal itself. As mentioned above, this could includethe received frequency (f_(r)) of the received signal, or any otherproperty of the received signal impacted by the Doppler effect.

The obtained information is compared at step S706 with one or moreexpected values, allowing an indication of the Doppler effect on thereceived signal to be determined, and thus an adjusted transmissionfrequency (f_(t)) to be determined at step S708. Then, the antennasystem 105 transmits the adjusted transmitted signal with transmissionfrequency (f_(t)).

FIGS. 8 and 9 show in more detail some of the elements that may bepresent in the antenna system 105 and communication control circuitry110 of the vehicle terminal 100, in accordance with the exampledescribed with reference to FIG. 7; in particular, FIGS. 8 and 9describe components to be used in a system in which an indication of thereceived frequency (f_(r)) of the received signal is used to perform theDoppler adjustment process. FIG. 8 shows elements present in a vehicleterminal 100 to be used in a time division duplex (TDD) scheme, in whichthe predetermined uplink frequency and predetermined downlink frequencyare the same while FIG. 9 shows an alternative arrangement to be used ina frequency division duplex (FDD) scheme, in which the predetermineduplink frequency and predetermined downlink frequency may be different.As noted above, the predetermined downlink frequency is the frequencywith which the ground terminal transmits the downlink signal, and thepredetermined uplink frequency is the frequency at which the groundterminal expects to receive the uplink signal.

In FIG. 8, the received signal Rx is received at an antenna 800, thereceived signal having a frequency equal to the carrier frequency(f_(c)) (the downlink frequency) adjusted according to the Dopplereffect (f_(c)+Δf). The received signal is fed into a frequency mixer802, the output of which is fed into a low pass filter (LPF) 804. Thefrequency estimator 806 estimates the frequency of the received signalbased on the output of the LPF 804, and supplies the signal to abaseband receiver 808. The frequency estimator 806 also supplies acontrol voltage to a reference oscillator 810, to cause the referenceoscillator 810 to then output a reference signal at a frequency(F_(ref)+ΔF_(ref)). The reference signal is fed into a local oscillator812, which multiplies the reference frequency by an upscaling factor aand outputs the resulting signal—corresponding to an estimation of thefrequency of the received signal—back into the frequency mixer 802. Theupscaling factor a is determined based on an RF upconversion controlsignal received at the local oscillator 812 from an RF controller 814.Thus, the above process implements a feedback loop, and the frequencyestimated by the frequency estimator 806 is more accurate with everypass.

The signal output by the reference oscillator 810 is also fed into acounter 816. A timing signal, received at a GPS antenna 818 andprocessed by a GPS element 820 is also fed into the counter 816. Thetiming signal provides one pulse per second (PPS), and hence, using thetiming signal, the counter 816 can count the number of cycles per secondin the reference signal output by the reference oscillator 810.

The counter 816 feeds into a logic circuit 822, controlled by the RFcontroller 814, which determines a downscaled adjustment value(2Δf_(ref)). The downscaled adjustment value (2Δf_(ref)) and the outputof a baseband transmitter 826 (having a frequency of F_(s)) are then fedinto a second frequency mixer 824.

The second frequency mixer 824 then outputs a signal (F_(s)−2Δf_(ref))to a third frequency mixer 828. The third frequency mixer 828 alsoreceives an input from the local oscillator 812 (i.e. a signalrepresenting the received frequency), and outputs a signal withfrequency F_(c)−2Δf, which is the adjusted transmitted frequencydescribed in earlier figures. This signal can then be transmitted as theuplink signal by an antenna 830.

The arrangement shown in FIG. 9 is almost identical to that shown inFIG. 8, with one main difference: the arrangement in FIG. 9 alsoincludes a second local oscillator 902, which receives signals from thereference oscillator 810 and the RF controller 814. This allows for thesignal fed into the third frequency mixer 828 (F_(c_ul)+Δf__(dl)) totake into account the difference in frequency between the uplink anddownlink signals in an FDD system.

It should be noted that the frequency of the transmitted signal isadjusted by a value of 2Δf, regardless of whether or not the downlinkfrequency f_(DL) and the uplink frequency f_(UL) are the same. This canbe shown as follows:

The Doppler frequency on the Forward Link (FL) is given by:

${\Delta \; f^{FL}} = {\frac{r \cdot v}{{r}c}f_{c}^{FL}}$

where f_(c) ^(FL) denotes the centre frequency on the forward link(downlink), c is the speed of light, v is the velocity vector and r isthe relative distance to the base station. The (·) symbol denotes thedot product operator, whereinr·v=(r_(x),r_(y),r_(z))·(v_(x),v_(y),v_(z))=r_(x)v_(x)+r_(y)v_(y)+r_(z)v_(z).

The Doppler frequency on the Reverse Link (RL), assuming that thecarrier frequency is f_(c) ^(RL), is given by:

${\Delta \; f^{RL}} = {\frac{r \cdot v}{{r}c}f_{c}^{RL}}$

The reference oscillator will therefore converge to:

$f^{REF} = {{\left( {f_{c}^{FL} + {\Delta \; f^{FL}}} \right)\text{/}\alpha^{FL}} = {\left( {1 + \frac{r \cdot v}{{r}c}} \right)f_{c}^{FL}\text{/}\alpha^{FL}}}$

where α^(FL) denotes the upscaling (multiplicative) factor for theforward link. For example, if f^(REF)=40 MHz, the a^(FL)=60 to ensurethe centre frequency will be at 2.4 GHz.

The received frequency at the base station (ground terminal) will bemultiple of the reference frequency (f^(REF)a^(RL)), adjusted by theDoppler effect. That is

$\begin{matrix}{f^{{RX} - {BS}} =} & {{{\left( {1 + \frac{r \cdot v}{{r}c}} \right)f^{REF}\alpha^{RL}} =}} \\ & {{\left( {1 + \frac{r \cdot v}{{r}c}} \right)\left( {1 + \frac{r \cdot v}{{r}c}} \right)f_{c}^{FL}\frac{\alpha^{RL}}{\alpha^{FL}}}} \\{=} & {{{\left( {1 + {2\frac{r \cdot v}{{r}c}} + \left( \frac{r \cdot v}{{r}c} \right)^{2}} \right)f_{c}^{FL}\frac{\alpha^{RL}}{\alpha^{FL}}} \approx}} \\ & {{\left( {1 + {2\frac{r \cdot v}{{r}c}}} \right)f_{c}^{FL}\frac{\alpha^{RL}}{\alpha^{FL}}}}\end{matrix}$ $\left( \frac{r \cdot v}{{r}c} \right)^{2} \approx 0$

since v²<<c². To prove this assumption, assuming 1000 km/h at 2.4 GHz,

${\left( \frac{r \cdot v}{{r}c} \right)^{2}f_{c}} = {0.002\mspace{14mu} {{Hz}.}}$

This is an insignificant contribution and can be ignored.

In TDD (time division duplex), a^(FL)=a^(RL) and f_(c) ^(FL)=f_(c)^(RL)=f_(c), which implies that Δf^(FL)=Δf^(RL)=Δf, and thusf^(RX−BS)=f_(c)+2Δf.

Note that in FDD (frequency division duplex) (or TDD),

${f_{c}^{RL} = {f_{c}^{FL}\frac{\alpha^{RL}}{\alpha^{FL}}}},$

thus

$f^{{RX} - {BS}} = {{\left( {1 + {2\frac{r \cdot v}{{r}c}}} \right)f_{c}^{RL}} = {f_{c}^{RL} + {2\mspace{14mu} \Delta \; f^{RL}}}}$

Therefore, just like in the TDD case we need to compensate thetransmission by 2Δf^(RL).

As shown through the above examples, the present technique allows thefrequency of a signal transmitted by a wireless communication systeminstalled in a fast-moving vehicle to be adjusted to compensate for theDoppler effect. This reduces interference effects at a ground terminal(base station), and allows higher frequency signals (such as those usedin modern telecommunications Standards) to be used. It also allows thesystem to be used in vehicles of increasing speeds. Thus, moderntelecommunications Standards such as 4G (LTE) can be implemented in ATGsystems, even as the speeds with which modern aeroplanes travel areever-increasing.

One of the functions performed by the communication control circuitry110 is to perform a sign-on procedure to seek to establish acommunication link with the ground terminal 130. During that sign-onprocedure, the communication control circuitry 110 will issue aconnection setup signal for receipt by the further antenna system 135within an identified timing window. The vehicle terminal 100 willfirstly receive an initial signal from the ground terminal 130 advisingof the availability for the connection setup signal to be issued, andproviding information regarding the identified timing window. The timingwindow will typically occupy one or more sub-frames, and the connectionsetup signal will have a duration less than the identified timingwindow, but will need to be received in its entirety within that timingwindow in order for a connection to successfully be established.

In accordance with the techniques described herein, it is assumed thatcommunications are taking place in accordance with the 4G (LTE)Standard, and such a connection setup signal may be referred to as aRACH (random access channel) signal that is issued in a random accesschannel during an uplink communication from the moving vehicle to theground terminal. Different RACH configurations may be supported, forexample associated with different sized RACH signals and associateddifferent sized timing windows.

FIG. 10A illustrates an example form of RACH configuration that could beused when adopting the communication frame format of FIG. 2, and insituations where the separation between the aircraft 10 and the groundterminal 20 does not exceed 108 km. Here the timing window occupiesthree sub-frames. As indicated by the communication frame 1000, it isassumed that the ground station 20 transmits a signal identifying thatthere is a RACH opportunity that the aircraft can utilise in an uplinkcommunication back to the ground terminal 20. As shown by the line 1005in FIG. 10A, the receipt of the communication frame at the aircraft 10is delayed by approximately 0.33 ms, due to the separation between theaircraft and the ground terminal (in this case it being assumed thatthere is essentially the maximum separation that can be supported usingthis RACH format). As shown by the line 1010, it is assumed that theaircraft 10 then transmits the RACH signal, in this case the RACH signalbeing propagated across all three of the uplink communicationsub-frames.

It will be appreciated that that uplink transmission will also bedelayed by the same propagation delay, and hence will be received by theground terminal 20 at approximately 0.66 ms delay (as indicated by theline 1015), due to the round trip delay between the ground terminal andthe aircraft. However, the timing control at the ground terminal isfixed, and hence it will assume the timing of the sub-frames is alignedwith the initial timing shown by the entry 1000. Hence, it willinterpret the received information on that basis.

In this case it is assumed that the RACH signal is received entirelywithin the RACH timing window, and based on the relative offset of thatRACH signal, the ground station 20 can identify that the totalpropagation delay is 0.66 ms. Accordingly, in a subsequent communicationframe 1020 where the ground station provides a response to identify thata successful communication link has been established, that responsesignal from the ground station will identify that the aircraft shouldadvance its timing for subsequent uplink communication by 0.66 ms. As aresult, this will ensure that the subsequent uplink communication isaligned with the sub-frame timing boundaries as implemented by theground terminal 20.

FIG. 10B illustrates the use of the same example RACH configuration, butin a situation where the separation exceeds the maximum separationdistance of 108 km. In this specific example, it is assumed that theseparation is 150 km resulting in a 0.5 ms propagation delay from theground terminal 20 to the aircraft 10. As shown by the line 1025, theground terminal 20 emits the same initial signal as discussed earlierwith reference to the line 1000 of FIG. 10A, and hence identifies a RACHopportunity. However, as shown by the line 1030, the communication frameis received after a 0.5 ms propagation delay. Again, as indicated by theline 1035, the aircraft terminal transmits the RACH signal within theuplink sub-frames, but again the communication is delayed by another 0.5ms on its transit to the ground terminal. Hence, there has been anoverall delay of 1 ms, and this results in the RACH signal not fallingwithin the RACH timing window, when using the timing adopted by theground station 20, as indicated by the line 1040. Accordingly, asindicated by line 1045, the RACH signal has not been successfullyreceived, and the ground station 20 will not send a response to theaircraft, as a result of which a communication link will not beestablished.

In accordance with the techniques described herein, this problem isaddressed by enabling the vehicle terminal to assess the separationbetween it and the ground terminal with which it is seeking to establisha communication, and to apply an initial timing advance relative to thedefault time indicated for the RACH signal, when issuing that RACHsignal to the ground terminal. This can be used to ensure that the RACHsignal is received within the specified timing window, hence enabling asuccessful communication link to be established. This process isdiscussed in more detail with reference to the flow diagram of FIG. 11.

As shown in FIG. 11, at step 1050 the vehicle terminal 100 observes asignal from the ground terminal 130 advising of the availability for theissuance of a connection setup signal (a RACH signal). This informationreceived by the vehicle terminal 100 also provides information about thedefault timing for issuing the RACH signal, the format of the RACHsignal, and the format of the timing window.

At step 1055, the distance computation circuitry 120 obtains thelocation information for the ground terminal, and determines aseparation distance between the vehicle terminal and the groundterminal. As discussed earlier, the distance computation circuitry 120may refer to the storage 125 in order to obtain the coordinates of theground terminal, based on that ground terminal's identifier includedwithin the communication from the ground terminal, and can obtaininformation about the location of the vehicle terminal from the GPSreceiver 115, hence enabling the separation distance to be determined.

At step 1060, it is determined whether the separation distance exceeds asetup threshold distance. If it does not, then the process proceeds tostep 1065, where the connection setup signal is sent in the standardmanner at the default timing, as per the process discussed for exampleearlier with reference to FIG. 10A. The setup threshold distance willdepend on the RACH configuration used, i.e. the format of the RACHsignal, and the size of the timing window, and the setup thresholddistance will be determined not to have been exceeded if the separationdistance is such that the RACH signal will be successfully received bythe ground station if merely transmitted at the default timing specifiedby the signal received at step 1050.

However, if at step 1060 it is determined that the separation distanceexceeds the setup threshold distance, then at step 1070 an initialtiming advance is chosen based on that separation distance. There are anumber of ways in which that initial timing advance can be determined,and one approach will be discussed later with reference to FIG. 14.

Once the initial timing advance has been determined at step 1070 then atstep 1075 the RACH signal is sent in the RACH channel at a timing basedon the initial timing advance. In particular, the default time isadjusted by the initial timing advance so that the RACH signal is issuedahead of the default time.

Due to the way in which the timing advance is chosen at step 1070, itwill hence be ensured that the RACH signal will be received within theRACH timing window by the ground station 130 even though the separationdistance exceeds the setup threshold distance.

Following either step 1065 or step 1075, the process proceeds to step1080, where the vehicle terminal 100 waits to see if a response isreceived from the ground terminal before a timeout period has elapsed.In particular, even though the RACH signal will have been receivedwithin the required timing window, it is not guaranteed that the groundterminal will choose to establish a communication link with the vehicleterminal. For example, it may be that the vehicle terminal is contendingwith a number of other vehicle terminals to establish a communicationlink, and the ground terminal may choose to establish a communicationlink with one or more of those other vehicle terminals instead of thecurrent vehicle terminal. For instance, certain vehicle terminals may begiven priority over others, and hence it may be that the vehicleterminal being considered in FIG. 11 does not obtain a communicationlink at that time.

If the ground terminal chooses not to establish a communication link, itwill not send a response back to the vehicle terminal, and accordinglyif such a response is not received within a certain timeout period, theprocess proceeds to step 1090 where the vehicle terminal will wait toretry establishing a communication link.

It may be that at step 1090 the vehicle terminal waits for another RACHopportunity to be identified by the same ground terminal, and thenretries establishing a communication link with that ground terminal. Itcould at that time take certain steps to increase the likelihood of itbeing allocated a communication link, such as for example increasing thepower of the transmission so as to indicate to the ground terminal thata better quality communication link could be established. For example,in one implementation, the vehicle terminal estimates path loss andcomputes an initial RACH power for detection, selects a preamble from anavailable set of preambles and transmits it. If that RACH request is notsuccessful, the vehicle terminal may autonomously choose another randompreamble and increase its power for the next RACH opportunity. This cancontinue until the vehicle terminal's maximum transmit power has beenreached.

However, the vehicle terminal is not limited to retrying to make aconnection with the same ground terminal, and if it receives an initialsignal from another ground terminal providing a connection setupopportunity, it could then seek to repeat the process of FIG. 11 inorder to establish a link with that ground terminal.

If at step 1080 it is determined that a RACH response is received fromthe ground terminal, hence identifying that the ground terminal hasaccepted the establishment of a communication link with the vehicleterminal, then the communication control circuitry 110 within thevehicle terminal 100 will analyse the response in order to determine howto control subsequent communication with the ground terminal. Inparticular, a further timing advance may be specified in the responsewhich should be used in combination with the initial (coarse) timingadvance chosen at step 1070 to control the timing of subsequent uplinkcommunication to the ground terminal. In addition, the response willtypically provide information about which sub-frames are allocated tothe vehicle terminal for downlink and uplink communications, so that thevehicle terminal can receive downlink communications destined for it asissued by the ground terminal 130, but can also issue its uplinkcommunications within an appropriate sub-frame, using the cumulativetiming advance determined at step 1085 so as to ensure that those uplinkcommunications are received at the appropriate timing by the groundterminal 130.

It should be noted that while the information in the RACH response isused to provide a fine timing advance that can be combined with thecoarse timing advance to determine the actual timing advance to be usedfor a subsequent uplink communication, as time progresses after thecommunication link has been established the distance between theaircraft and the ground terminal will change. This change can becompensated for using standard techniques provided by the 4G (LTE)Standard to make fine timing adjustments during the duration of thecommunications link.

FIG. 12A illustrates how the process of FIG. 11 is applied for aparticular implementation of the RACH signal and RACH timing window. Inthis example, it assumed that the RACH timing window is specified ascoinciding with the third uplink communication sub-frame (U2), and thatthe RACH signal as transmitted will need to land entirely within thatsub-frame in order for a successful communication to be established. Asindicated by the line 1100, the ground station transmits a signalidentifying the RACH opportunity that can be used within the uplinkpath. As indicated by the line 1105, due to the separation between theground terminal 130 and the vehicle terminal 100, which in this case isassumed to be the maximum allowable distance of 300 km, the vehicleterminal 100 receives the communication frame delayed by 1 ms, and hencethe communication frame is offset by a sub-frame width.

As indicated by the line 1110, because the separation distance exceedsthe setup threshold distance at step 1060, an initial timing advance ischosen at step 1070 based on the separation distance, and in this casethat initial timing advance will be chosen to be 2 ms. A full 2 msadvance can be applied without risk of violating a receive/transmittiming constraint, since even when the RACH signal is advanced by 2 ms,the vehicle terminal is not seeking to transmit that RACH signal at atime when it should be configured for receiving downlink communication,as is evident by the line 1110.

As indicated by the line 1115, that RACH signal will then actually bereceived with a 1 ms delay relative to its transmission time, which thenrealigns the RACH signal with the RACH timing window. Accordingly, theconnection setup signal (the RACH signal) will be received, andaccordingly a communication link can be established.

Assuming the ground terminal determines that a communication link is tobe established with the vehicle terminal, then it will transmit acommunication frame 1120 as a RACH response, which will be received witha 1 ms delay, as indicated by the line 1125. This can specify a finetiming advance if needed, which can be applied in combination with thecoarse timing advance applied by the vehicle terminal to controlsubsequent uplink communications. The RACH response will also typicallyprovide an indication of which sub-frames are allocated to the vehicleterminal for subsequent downlink and uplink communications.

As indicated in FIG. 12B, it is assumed in this instance that thevehicle terminal is allocated as its uplink sub-frame the sub-frame U2,and will accordingly perform an uplink transmission at a timingindicated by the line 1130 for its subsequent uplink communications. Asindicated by the line 1135 in FIG. 12B, due to the timing advanceapplied, this will ensure that the uplink communication is actuallyreceived at the correct timing by the ground terminal 130.

It should be noted that whilst in FIG. 12A it is assumed that the RACHconfiguration specifies that the RACH timing window is associated withthe U2 sub-frame, as discussed earlier different RACH configurations canbe used. For example, a RACH configuration may be used where the timingwindow is associated with both the U1 and the U2 sub-frames, with alonger RACH signal being issued, but with the requirement that a RACHsignal lands in its entirety within the U1 and U2 sub-frames as per thetiming adopted by the ground terminal 130. In another example, the RACHconfiguration may specify the use of all three uplink sub-frames as theRACH timing window, again with a longer RACH signal, but again with therequirement that that RACH signal lands entirely within the timingwindow as per the timing adopted by the ground terminal 130. The choiceof RACH configuration will affect the setup threshold distance that isassessed at step 1060 of FIG. 11, and may affect the initial timingadvance that is then chosen at step 1070 in situations where thedistance exceeds the setup threshold distance.

For instance, whilst in the example of FIG. 12A the initial timingadvance chosen based on the separation distance does not have to beconstrained to take into account the requirement not to violate areceive/transmit timing constraint, with other RACH configurations theinitial timing advance chosen may need to be constrained so as to ensurethat the receive/transmit timing constraint is not violated. Forexample, it will be appreciated that if the RACH timing window occupiesboth the U1 and the U2 sub-frames, and a 2 ms advance was applied as perthe example shown in FIG. 12A based on a separation distance of 300 km,this means that the transmission of the RACH signal will overlap withthe S0 sub-frame. However, the receive/transmit timing constraint wouldthen be violated if such an advance resulted in the need to transmit anuplink signal whilst the antenna system 105 should still be configuredfor downlink communication. In addition to the fact that it takes afinite time to perform the switch, as mentioned earlier it is alsopossible that some of the first symbols within the S0 sub-frame may beused for downlink communication, and accordingly in that instance it maynot be appropriate to fully advance the initial timing by the timingthat would be determined based purely on the propagation delay. Instead,it may be necessary to choose a slightly smaller coarse timing advanceto avoid violating the receive/transmit timing constraint, whilstensuring that that timing advance is sufficient to cause the RACH signalto be received within the RACH timing window. The further timing advancedetermined by the ground terminal will then compensate for the initialtiming advance, so that cumulatively the initial and further timingadvances will provide the required timing advance for subsequent uplinkcommunication.

FIG. 13 is a flow diagram illustrating one way in which step 1055 ofFIG. 11 may be performed. In this example, it is assumed that theinitial communication from the ground station includes a ground stationidentifier. At step 1150, the distance computation circuitry 120extracts that ground station identifier from the received signal, andthen at step 1155 performs a lookup in the database provided within thestorage 125 in order to obtain the location coordinates for the groundstation.

At step 1160, the distance computation circuitry 120 then obtainslocation coordinates of the vehicle terminal 100 from the GPS receiver115, and thereafter at step 1165 computes the separation distancebetween the ground terminal and the vehicle terminal.

Whilst the approach of FIG. 13 can be used in one exampleimplementation, in an alternative implementation it may be that theinitial signal from the ground terminal directly provided thecoordinates of the ground terminal, and accordingly those coordinatescould be extracted from the received signal at step 1150, and no lookupin the database would be required (hence step 1155 becoming redundant).

FIG. 14 is a flow diagram illustrating how step 1070 of FIG. 11 may beperformed in one example implementation. At step 1200, it is determinedwhich range of separation distances the separation distance fallswithin. Then, at step 1205 a timing advance appropriate for that rangeis determined. For instance, it could be that a lookup table is usedthat provides suitable coarse timing advances to be used for each of anumber of different ranges. That lookup table could provide timingadvances applicable for a number of different RACH configurations (i.e.for different formats of RACH signal and RACH timing window), with thelookup operation obtaining the timing advance appropriate for thedetermined range and RACH configuration.

However, in some implementations it may be determined that a lookuptable approach based on ranges is not required, and instead theseparation distance may be determined on the fly. In particular, aninitial timing advance can be determined by dividing the separationdistance by the speed of light.

As shown in FIG. 14, the process then proceeds to step 1210, where it isdetermined whether there is any receive/transmit timing violation issue.As discussed earlier, this may depend on the RACH configuration used andthe separation distance in question. In particular, for RACHconfigurations that use multiple sub-frames, it may be the case thatwhen the separation distance exceeds a certain amount, then there couldbe a receive/transmit timing violation issue if the timing advancedetermined at step 1205 was used “as is”.

If it is determined that there is not any receive/transmit timingviolation issue, then the process proceeds to step 1215 where thedetermined timing advance evaluated at step 1205 is used.

However, if it is determined that there is a receive/transmit timingviolation issue, then at step 1220 the timing advance can be scaled backto ensure that the receive/transmit timing constraint is not violated,whilst still enabling receipt of the connection setup signal within thetiming window.

In instances where the timing advance is encoded within a lookup tablebased on ranges of separation distance, then as mentioned earlier in oneexample implementation that lookup table will provide timing advanceinformation for each of a number of different possible RACHconfigurations, and the prospect of violating receive/transmit timingconstraints can be taken into account when populating the lookup table,so that in effect the evaluation at step 1210 is taken into account wheninitially populating the lookup table. In that event it will merely besufficient to determine the range that the separation distance fallswithin and then obtain the appropriate timing advance to use from thelookup table at step 1205. Hence, in that case steps 1210, 1215 and 1210would not be needed.

In one example implementation, when determining the appropriate timingadvance to use, the aim is to try and land the connection setup signalwithin the middle of the specified timing window. By such an approach,this can allow for any inaccuracy in the timing advance applied, toensure not only that the entire connection setup signal is receivedbefore the end of the timing window, but also that no portion of thatconnection setup signal is received before the start of the timingwindow.

It should be noted that the above coarse timing advance scheme can beapplied to a wide variety of different communication schemes, forinstance both TDD (time division duplex) and FDD (frequency divisionduplex) schemes. When employing an FDD scheme, the above-mentionedreceive/transmit timing constraint issue may not apply as the antennasystem can transmit and receive simultaneously, and hence steps 1210 and1220 of FIG. 14 will not be employed.

Using the above described techniques, it is possible to establish acommunication link with the ground terminal, even in situations wherethe separation distance between the aircraft 10 and the ground terminal20 exceeds that supported using the standard RACH mechanism. However, asillustrated schematically in FIG. 15, a further problem that can ariseis ensuring that in the subsequent uplink communications from theaircraft to the ground station 10 (using the cumulative timing advanceobtained by combining the initial timing advance chosen by the vehicleterminal 100 with the fine timing advance specified in the RACHresponse), the earlier-mentioned receive/transmit timing constraint isnot violated. In particular, as shown in FIG. 15, the communicationframe format provides multiple sub-frames that can in principle be usedfor uplink communication, namely the sub-frames U0, U1 and U2 shown inthe communication frame 1250. However, as indicated by the combinationof the lines 1255 and 1260, if the scheduling circuitry 140 within theground terminal 130 chooses to allocate resource blocks to the aircraft10 within either the U0 or the U1 sub-frames, then if the aircraftseparation distance from the ground terminal exceeds a schedulingthreshold distance (in this example the scheduling threshold distancebeing 100 km), then the receive/transmit timing constraint would beviolated.

In the example of FIG. 15, it is assumed that the separation distancebetween the aircraft 10 and the ground terminal 20 is 300 km, and hencefrom the earlier discussed FIG. 12A it will be understood that a timingadvance of approximately 2 ms may be specified. However, this wouldoverlap the sub-frames U0 and U1 with the downlink sub-frame D0 and thespecial sub-frame S0, and as discussed earlier the special sub-frame S0may include some symbols transmitting downlink information. At any pointin time, the antenna system 105 can only be configured for downlinkcommunication or uplink communication, so this would violate thereceive/transmit timing constraint, even though, as indicated by theline 1265, that timing advance would correctly align the uplinkcommunications so that they are received in the relevant sub-frames U0,U1, U2 as per the timing employed by the ground terminal 130.

FIGS. 16A and 16B provide a flow diagram illustrating steps that can beperformed by the ground terminal when determining how to schedulesub-frames to the vehicle terminal, in order to resolve the issueillustrated in FIG. 15. At step 1300, the ground terminal will awaitreceipt of a connection setup signal, i.e. the earlier discussed RACHsignal, from the vehicle terminal. Then, at step 1305 the groundterminal determines whether to allow the vehicle terminal 100 toestablish a communication link with it. As discussed earlier, a numberof criteria can be assessed here. For example, the quality of thecommunication link can be assessed, and factors such as other vehicleterminals that are seeking to establish a communication link can beconsidered when deciding whether to accept the establishment of acommunication link with the vehicle terminal 100.

At step 1310, it is then concluded whether a communication link is to beestablished or not, and if not then at step 1315 the connection setuprequest is merely ignored. As will be apparent from the earlierdiscussed FIG. 11, this will result in no response being received by thevehicle terminal within a specified timeout period, and accordingly thevehicle terminal will proceed to step 1090 in order to seek to establisha communication link at a future time, either with that ground terminal130, or with another ground terminal.

Assuming it is decided at step 1310 that a communication link is to beestablished, then at step 1320 the communication link establishing andscheduling circuitry 140 computes a timing advance required based on thereceived connection setup signal. In particular, based on the placementof the received RACH signal within the RACH timing window, a timingadvance can be computed, this being the fine timing advance discussedearlier. At this stage, the computation performed by the communicationlink establishing and scheduling circuitry 140 does not need to takeaccount of the actual separation distance between the aircraft and theground terminal, since as discussed earlier that fine timing advancewill be combined with any coarse timing advance initially chosen by theaircraft when sending the RACH signal, in order to determine the fulltiming advance to be used for subsequent uplink communication.

However, as discussed earlier care needs to be taken when schedulinguplink sub-frames for the aircraft to ensure that the receive/transmittiming constraint is not violated, and to assist in this process theground terminal 130 does need to determine the separation between thevehicle terminal 100 and the ground terminal.

Accordingly, at step 1325 the ground terminal is arranged to determinethe location of the vehicle terminal. In particular, the distancecomputation circuitry 145 discussed earlier in FIG. 3 can accessinformation in order to determine the current position of the aircraft10. There are a number of ways in which the vehicle location informationcan be obtained, but in one example a flight tracking website may beaccessed in order to obtain current coordinate information. Thereafter,at step 1330 the separation distance between the ground terminal and thevehicle can be determined. In particular, the location of the groundterminal 130 will be fixed, and accordingly can be used when computingthe separation distance.

Then, at step 1335, one or more uplink sub-frames are allocated for useby the vehicle terminal taking into account the separation distance, soas to avoid violation of the receive/transmit timing constraint. Inparticular, in one example arrangement there may be multiple sub-framesthat can be allocated for uplink communication, such as the threesub-frames U0, U1, U2 discussed earlier. Which of those sub-frames isused when allocating uplink resource for the aircraft 10 can takeaccount of the separation distance. This will be discussed in moredetail later by way of example with reference to FIGS. 17A to 17C.However, from the earlier-discussed FIG. 15, it will be appreciated thatin the particular example chosen in FIG. 15 the scheduling circuitrycould avoid allocating resource blocks within the sub-frames U0 and U1,so that the aircraft is only allocated resource blocks within thesub-frame U2, such that when the timing advance is applied thereceive/transmit timing constraint will not be violated.

As indicated at step 1340, downlink sub-frames are also allocated to beused by the vehicle terminal for downlink communication from the groundstation to the aircraft.

Once the uplink and downlink sub-frames have been allocated, then theresponse signal can be issued to the vehicle terminal at step 1345identifying both the timing advance determined earlier at step 1320, andthe uplink and downlink sub-frames that are to be used for subsequentcommunication with the aircraft.

FIGS. 17A to 17C illustrate how uplink resource can be scheduled,assuming the communication frame format is as discussed earlier in FIG.2, and accordingly there are three sub-frames that can in principle beused for uplink communication. As indicated in FIG. 17A, where it isdetermined that the aircraft 10 is at 300 km from the relevant groundterminal 20, the propagation delay is 1 ms, and accordingly thecommunication frame 1350 as transmitted by the ground terminal isreceived as shown by the line 1355, such that the communication is onesub-frame out relative to the transmission timing. In this example, itis assumed that the scheduling circuitry determines at step 1335 toallocate the U2 sub-frame to the vehicle terminal for use in uplinkcommunication. As a result, as indicated by the line 1360, when thecumulative timing advance of 2 ms is applied, the downlink/uplink timingconstraint is not violated. Hence, the uplink communication can beperformed using this timing advance, and will ensure that it iscorrectly received by the ground terminal in the U2 sub-frame, asindicated by the line 1365. The approach shown in FIG. 17A can be usedwherever the separation distance exceeds 200 km, provided the separationdistance does not exceed 300 km.

FIG. 17B illustrates a scheduling approach that can be used when theseparation distance is between 100 and 200 km. Again, the communicationframe 1370 is transmitted from the ground terminal 20, and in thisspecific example it is assumed that the separation is 150 km, and hencethe delay in receiving the communication frame is 0.5 ms as shown by theline 1375. In this scenario, the cumulative timing advance that willapplied after the RACH sign-up process has been completed will be 1 ms.As a result, it is possible to accommodate uplink allocations in eitheror both of sub-frames U1 and U2 without violating the downlink/uplinktiming constraint, as indicated by the line 1380. As shown by the line1385, uplink communications in either of those two sub-frames will thenbe correctly received by the ground terminal 20.

FIG. 17C illustrates a scheduling scheme that can be used when theseparation distance is less than 100 km. The communication frame 1390 istransmitted from the ground terminal, and in this instance it is assumedthat the separation delay is 0.17 ms, this assuming the separationdistance is 50 km. In this instance, any of the three uplink sub-framesU0, U1 or U2 can be allocated for uplink communication, since thecumulative timing advance after the RACH process has been performed willbe 0.33 ms.

As shown by the line 1400, if the sub-frame U0 is used, this will causesome overlap of the U0 sub-frame transmission timing with the S0 frame.However, the extent of overlap still leaves some gap, and in particulardoes not overlap with any symbols within the S0 sub-frame that will beused for downlink communication, and accordingly the receive/transmittiming constraint is not violated. Further, as shown by the line 1405,any uplink communication of the three sub-frames U0, U1 or U2 will becorrectly received by the ground terminal with the appropriate timing.

It is anticipated that the traffic between an aircraft and a connectedground terminal will be heavily downlink centric, for example to supportthe earlier-mentioned Wi-Fi connectivity for passengers within theaircraft. As will be apparent from the earlier-discussed frame format ofFIG. 2, when using that frame format three sub-frames are reserved foruplink communication. This is required to allow for effective schedulingof uplink communications for aircrafts up to 300 km away from the groundterminal. However, in one example implementation the base station may beprovided with the flexibility to alter the communication frame formatunder certain conditions, in order to allow for a larger proportion ofthe communication frame to be used for downlink traffic when possible.

FIG. 18 illustrates three example communication frame formats that maybe used, each of which are supported LTE TDD (Time Division Duplex)frames. The frame format FC3 1410 is the format discussed earlier withreference to FIG. 2. The format FC4 1415 has one less uplink sub-frameand one more downlink sub-frame. Further, the frame format FC5 1420 hasonly a single uplink sub-frame, and an additional downlink sub-framerelative to the frame format FC4.

From the earlier scheduling examples illustrated with reference to FIGS.17A to 17C, it will be appreciated that it is only when the separationdistance exceeds 200 km (referred to in FIG. 18 as long range (LR)) thatthere is a need to schedule uplink communication in the last of thethree uplink sub-frames, and hence the requirement to use communicationframe FC3. When the distance is between 100 and 200 km (referred to inFIG. 18 as medium range (MR)), then uplink communication can bescheduled in the second uplink sub-frame, and hence it would still bepossible to schedule uplink communications even if the communicationframe format FC4 was used. Similarly, it will also be appreciated thatif the communication frame format FC4 is used, uplink communication withaircraft up to 100 km away (referred to in FIG. 18 as short range (SR))can also be accommodated when using the communication frame format FC4.

Finally, it will be appreciated that if the aircraft is less than 100 kmaway, then the communication frame format FC5 could be used, sinceuplink communication can be scheduled in the first uplink sub-frame(which happens to be the only uplink sub-frame in the frame format FC5).

FIG. 19 is a flow diagram illustrating how the ground terminal couldmake use of the three communication frame formats shown in FIG. 18 inorder to facilitate a higher downlink capacity when the location of theconnected aircrafts permits. At step 1450, it is determined whether allof the aircraft connected to that ground station are within the mediumor short ranges. If not, then the communication frame FC3 is used atstep 1455, and the process returns to step 1450.

However, if all of the connected aircraft are within the medium or shortrange, then the process can proceed to step 1460 where the aircraftterminal can switch to using communication frame FC4. A broadcast signalcan be sent from the ground terminal to all of the connected aircraftterminals to advise them of the change in the communication frame. Oncestep 1460 has been implemented, it will be appreciated that there is anadditional downlink sub-frame available when compared with thecommunication frame FC3.

Following step 1460, it can be determined at step 1465 whether allconnected aircraft are within the short range. If not, it is thendetermined at step 1470 whether there is a desire to connect with anaircraft exceeding the medium range. For example, the ground terminalmay receive a RACH signal from an aircraft within the long range seekingto establish a connection, and the ground terminal may decide that itwishes to service that request. Alternatively, it may be known that oneof the already connected aircraft is about to leave the medium rangeinto the long range, and it may be desirable to maintain connection withthat aircraft. If it is determined at step 1470 that there is desire toconnect with an aircraft exceeding the medium range, then the processproceeds to step 1455 where a switch is made to using the communicationframe FC3. Again, a broadcast signal can be sent from the ground stationto identify this change in the communication frame.

However, if at step 1470 it is determined that there is no desire toconnect with an aircraft exceeding the medium range, then the processcan merely return to step 1460.

If at step 1465 it is determined that all of the connected aircraft arewithin the short range, then the process can proceed to step 1475 wherethe communication frame FC5 can be used. Again, a broadcast signal canbe sent from the ground terminal to advise of the change in thecommunication frame format.

Following step 1475, it can be determined at step 1480 whether there isa desire to connect with an aircraft exceeding the short range. If not,the process merely returns to step 1475 where the communication frameformat FC5 continues to be used. However, if at step 1480 it isdetermined that there is a desire to connect with an aircraft exceedingthe short range, then the process proceeds to step 1470 where theearlier-discussed analysis is performed.

Accordingly, by such an approach, it can be seen that the groundterminal can make use of multiple communication frame formats so as toseek to maximum the downlink capacity available, taking into account theseparation between that ground terminal and the relevant aircraft. Thiscan further improve capacity within the network.

In one example implementation where lookup tables are used to determineinitial timing advances to be applied for RACH signals, those lookuptables can be updated as necessary dependent on the communication frameformat currently being employed by the ground terminal.

From the above described examples, it will be seen that the techniquesdescribed herein enable for a timing adjustment to be made within awireless communication system for a moving vehicle to enable wirelesslinks to be established between the moving vehicle and a groundterminal, even when the separation distance between the moving vehicleand the ground terminal exceeds the maximum separation distancesupported by the sign-on procedure when using the wireless communicationStandard provided within the wireless network. Further, once such a linkhas been established, the scheduling of uplink resource to the aircraftcan be adapted so as to ensure that receive/transmit timing constraintsare not violated, even in situations where the separation distanceexceeds the maximum separation distance supported by thetelecommunications Standard.

In the above described examples, adjustments to the frequency or timingof signals transmitted by a vehicle terminal of a moving vehicle arecalculated by circuitry within the vehicle terminal itself. However, theinventors realised that in some situations it may be beneficial not toimplement such mechanisms within the vehicle terminal itself, butinstead to use a centralised mechanism to perform such computations.This will allow, for instance, computations to be made for base stationsto which the moving vehicle is not currently connected, for example toassist in the handover procedure. Moreover, even if the above techniquesare implemented, it may be beneficial to employ an additional mechanismto aid the above technique. The following examples provide such amechanism.

Similarly to FIG. 1, FIG. 20 shows an aircraft 10 in communication witha base station (ground terminal) 20, and travelling at a velocity 40, ata given angle 50 to a line connecting the vehicle and the base station.This base station 20 is the current base station, or connected basestation, and is one of a network of base stations configured tocommunicate with the aircraft. Also shown in FIG. 20 is another basestation 2000 of the network of base stations, that is not currently incommunication with the aircraft. However, the other base station 2000 isa handover candidate for selection as the next base station to which theaircraft 10 will connect. When it is determined that a better signalquality will be achieved by switching to the other base station 2000, ahandover procedure may be initiated to transfer communication with thevehicle terminal from the current base station 20 to the other basestation 2000.

As discussed above, the aircraft is provided with a vehicle terminal(also known as an air terminal or air station) for communication withthe network of base stations on the ground. The vehicle terminal, thebase stations and other components to be described herein, form acommunication network. FIG. 21 shows some of the components that may bepresent in a communication network 2100 according to the presenttechnique.

In the communication network 2100, an air terminal 2102 (also known asan air station or a vehicle terminal) is in communication with a currentbase station 2104. The current base station is one of a network of basestations 2104-2112, with some of the base stations in the networkforming a neighbourhood 2114 of base stations. The base stations2104-2108 in the neighbourhood 2114 are base stations which serve aspotential candidates for a handover procedure to transfer the airterminal 2102 from the current base station 2104 to a selected otherbase station in the network. Which base stations form the neighbourhood2114 may vary dependent on implementation, but may for example bedetermined with reference to the currently connected base station 2104.In some instances that default set of candidate base stations may bealtered taking into account other information such as the direction oftravel, and/or the bearing, of the aircraft.

In the example depicted in FIG. 21, one of the base stations 2108 hasbeen selected as the next base station, and a handover procedure is thusexpected to be performed to transfer communication with the air terminal2102 from the current base station 2104 to the selected other basestation 2108.

The base stations 2104-2112 are connected via a wired or wirelessconnection to the internet 2116.

Also shown in the communication network 2100 depicted in FIG. 21 is acommunication manager 2118 for managing communication between the basestations 2104-2112 and the air terminal 2102 in the communicationnetwork 2100.

The communication manager 2118 comprises a controller 2134 responsiblefor controlling the communication manager 2118 and an interface 2120 fortransmitting and receiving signals to or from the rest of the network.For example, the current base station 2104 may send offset measurementinformation indicative of measured differences between the expectedtiming and frequency of signals received at the base station 2104, andthe actual timing and frequency of the received signals. The currentbase station 2104 transmits this offset information via the internet2116 to the interface 2120. However, it should be noted that in otherexamples, offset information is not transmitted to the interface 2120from the current base station 2104. This is discussed in more detailbelow, with reference to FIGS. 23 to 27.

The communication manager also includes correction determinationcircuitry 2122 for calculating frequency and/or timing correctioninformation for signals to be transmitted from the air terminal 2102 tothe base stations 2104-2108 in the neighbourhood 2114.

The correction determination circuitry 2122 is configured to determinethe timing and/or frequency corrections based on information about thebase stations 2104-2108 and the aircraft. For example, the correctiondetermination circuitry is configured to obtain vehicle trackinginformation from an aviation database 2124, accessed by aviationdatabase access circuitry 2126 via the internet 2116 and the interface2120. The vehicle tracking information may include a location andvelocity of the aircraft in which the air terminal 2102 is installed.Meanwhile, location information about each base station 2104-2108 in theneighbourhood 2114 can be determined by accessing, via storage accesscircuitry 2128, a storage unit 2130, using a base station identifier asan input.

Using the vehicle tracking information and the base station locationinformation, distance computation circuitry 2132 is configured todetermine a separation distance (scalar or vector) between the airterminal 2102 and each of the base stations 2104 to 2108. Based on thisinformation, the correction determination circuitry 2122 is configuredto calculate, using the Doppler formula discussed above, frequencycorrections to be applied to signals to be transmitted from the airterminal 2102 to the base stations 2104-2108 in the neighbourhood 2114.The correction determination circuitry 2122 is also arranged todetermine timing corrections to be applied to those signals, independence on the calculated separation distance.

While the above example describes calculating frequency and timingcorrection information (transmission adjustment control information) forall of the base stations 2104-2108 in the neighbourhood 2114, it will benoted that in some examples transmission adjustment control informationis only determined for those base stations 2106, 2018 of theneighbourhood 2114 that are not currently in communication with the airterminal (e.g. the handover candidates).

Furthermore, when transmission adjustment control information iscalculated for the current base station 2104, it may be calculated inany of a number of different ways. One example is to use the techniquedescribed above, using vehicle tracking information and base stationlocation information. However, in some examples, the interface isconfigured to pass received offset information from the connected basestation to the correction determination circuitry 2122. The correctiondetermination circuitry 2122 then uses the offset information todetermine frequency and/or timing adjustment information to be appliedto signals transmitted from the air terminal 2102 to the connected basestation 2104. For example, the correction determination circuitry 2122may receive offset information for a plurality of signals received bythe current base station 2104 and calculate a filtered adjustmentestimate. This is expanded on below, when discussing FIGS. 23 to 27.

Regardless of the technique by which the transmission adjustment controlinformation is determined, and regardless of which base stations2104-2108 the information is calculated for, the information is passedto the controller 2134, which either passes it onto the interface 2120for transmission to the current base station 2104 or one of thenon-connected base stations 2106, 2108, or passes it onto the storageunit 2130 to be stored for later use. Once the transmission adjustmentcontrol information is received by the current base station 2104, it isfurther transmitted to the air terminal 2102, which applies thecalculated frequency and/or timing adjustment to signals it transmits.The transmission adjustment control information relating to other basestations in the neighbourhood 2114 may further be transmitted to thosebase stations, for use during handover of the air terminal 2102 to oneof those base stations.

Thus, according to the above example, frequency and/or timing correctioninformation is determined in a communication manager 2118 on the ground,rather than in the air terminal 2102. This can improve compatibility ofthe system with existing air terminals 2102 and base stations 2104-2112,which may not have the capability to calculate the adjustmentinformation themselves.

The above example can also be applied in addition to mechanisms by whichfrequency or timing corrections are determined by circuitry in the airterminal 2102. In this case, the correction determination circuitry maydetermine a correction which is relative to another correction alreadyapplied by the air terminal 2102 (relative adjustment controlinformation). Alternatively (or in the case where the air terminal 2102does not calculate any correction information itself), the correctiondetermination circuitry 2122 may determine a correction which as anabsolute correction, to be applied to a signal as generated by the airterminal 2102.

FIG. 22 is a flow diagram showing an overview of a method carried out bythe correction determination circuitry 2122 of FIG. 21 to determinetransmission adjustment control information for base stations 2104-2108in the neighbourhood 2114.

In step S2202, base station location information for base stations2104-2108 in the neighbourhood is obtained. As discussed above, thisinformation may be obtained by accessing, with storage access circuitry2128, a storage unit 2130 using at least one base station identifier.

In step S2204, moving vehicle tracking information of the moving vehicleis obtained. For example, this could be achieved by accessing anexternal aviation database 2124 (in the case where the vehicle is anaircraft).

In step S2206, transmission adjustment control information is determinedfor at least each non-connected base station 2106, 2108 in theneighbourhood 2114. As discussed above, the transmission adjustmentcontrol information may be frequency adjustment information indicativeof a frequency adjustment to be applied by the aircraft to atransmission frequency of a signal, so as to reduce a frequencydifference between an observed frequency of the signal at the selectedbase station and a predetermined uplink frequency, and/or it may betiming adjustment information indicative of a timing adjustment to beapplied to a transmission time of the signal, so as to reduce a timingdifference between a reception timing of the signal at the selected basestation and an expected timing. The timing correction may refer to acorrection to be applied to various signals, for example a RACH signalas described above with reference to FIGS. 12A and 12B, or moregenerally to support uplink scheduling as discussed in FIGS. 16A to 18.

Once the transmission adjustment control information has been determinedit may be transmitted, in step S2208, for reception by the vehicleterminal in the moving vehicle. Referring back to FIG. 21, thetransmitted adjustment control information is transmitted by theinterface 2120 of the communication manager 2118 to the current basestation 2104, to be transmitted to the vehicle terminal (the airterminal 2102 in FIG. 21). Whilst all of the generated adjustmentcontrol information can be transmitted if desired, in some instancesonly a subset of the generated information will be transmitted, forexample that related to a selected base station to which it has beendecided to perform a handover procedure.

A more detailed example of a method of determining transmissionadjustment control information is illustrated in FIG. 23. This methodmay, for example, be carried out in the communication manager 2118 ofFIG. 21.

In step S2302, the neighbourhood of base stations 2114 is identified.The neighbourhood 2114 may be identified in any of a number of ways. Forexample, the neighbourhood 2114 may be defined as all of the availablebase stations within a given distance of the current base station 2104.Alternatively, the selection of base stations to be included within theneighbourhood 2114 may be narrowed down further, based on other factorssuch as the bearing of the vehicle.

Once the neighbourhood 2114 of base stations has been identified, instep S2304 the next base station in the neighbourhood 2114 is selected.The selection of the “next” base station may be performed by anyapplicable method; for example, the next base station may be selectedrandomly, or it may be the base station with the next base stationidentifier in a chronological order.

Whichever method is used to select the next base station, once it isselected, the method proceeds to step S2306, in which it is determinedwhether or not the selected base station is the connected (i.e. current)base station 2104.

If, in step S2306, it is determined that the selected base station isthe current base station 2104, a determination S2308 is made as towhether or not offset information has been received for more than onesignal. If the answer at this step is “yes,” the method proceeds to stepS2310, and correction determination process A (to be discussed laterwith reference to FIGS. 24 to 25) is performed. On the other hand, ifthe answer at this stage is “no,” the method proceeds instead to stepS2312 and correction determination process B (to be discussed later withreference to FIGS. 26 to 27) is performed.

Returning to step S2306, if it is instead determined at this stage thatthe selected base station is not the connected base station, the methodproceeds to step S2312 and correction determination process B is carriedout.

Following the implementation of either correction determination process,the method proceeds to step S2314, in which it is determined whether theselected base station is the last base station in the neighbourhood2114. If the selected base station is not the last base station, themethod returns to step S2304 and repeats for the next base station. Onthe other hand, if the selected base station is the last base station,the method proceeds to step S2316.

In step S2316, in response to a trigger being detected, at least asubset of the correction information is transmitted to the current basestation 2104, for further transmission to the vehicle terminal. Thetrigger may take a variety of forms. For example, the trigger could be aspecific request transmitted to the communication manager by the currentbase station. Alternatively, the trigger may simply be a determinationthat the correction information for all of the base stations in theneighbourhood 2114 has been determined, or it may be the passage of apredetermined period of time.

Whatever the trigger, the correction information may be stored in thestorage unit 2130 in the communication manager 2118 before it istransmitted. This is illustrated in FIGS. 24 to 27.

Correction determination processes A and B will now be discussed withreference to FIGS. 24 to 27.

FIG. 24 is a flow diagram illustrating correction determination processA in a case where the correction information to be determined isfrequency correction information. As discussed earlier, with referenceto FIG. 23, this process S2310 a requires offset information for atleast two signals to be received by the communication manager 2118, andcan only be applied to the current base station 2104.

Correction determination process A starts with a step S2402 ofextracting, from the received offset information for the two or moresignals, frequency information. The frequency information is informationindicative of a difference between an observed frequency of each of theplurality of signals received at the current base station and apredetermined uplink frequency of that previous signal.

Then, in step S2404, the frequency correction 2Δf to be applied tosignals transmitted by the vehicle terminal is determined by filteringthe received offset information.

Finally, once the frequency correction has been determined, it is storedat step S2406 in the storage unit 2130.

It is noted that the filtered offset information does not need to bedoubled in order to obtain the frequency correction of 2Δf. Signalsreceived at the air terminal are offset by Δf due to the Doppler effecton those downlink signals, so that signals received at the base stationare offset by an additional Δf, giving them a 2Δf offset overall. Thus,the “double” is already incorporated in the offset information used todetermine the frequency correction.

FIG. 25 is a flow diagram illustrating a related process S2310 b to beapplied where the correction information to be determined is timingcorrection information. As with FIG. 24, this process S2310 b requiresoffset information for at least two signals to be received by thecommunication manager 2118, and can only be applied when the selectedbase station is the connected base station.

In this case, correction determination process A starts with a stepS2502 of extracting, from the received offset information for the two ormore signals, timing information. The timing offset information isinformation indicative of a difference between a reception timing ofeach of the plurality of previous signals at the current base stationand an expected timing for that previous signal.

Then, in step S2504, the timing correction to be applied to signalstransmitted by the vehicle terminal is determined by filtering thereceived offset information.

Finally, once the timing correction has been determined, it is stored atstep S2506 in the storage unit 2130.

It should be noted that, while frequency corrections and timingcorrections have been described as separate processes, it is alsopossible for frequency and timing corrections to be calculated together.For example, process A can be performed for both timing and frequencycorrections at the same time, by extracting both frequency informationand timing information together in steps S2402 and S2502 and using bothsets of information to compute filtered estimates of the corrections.

FIG. 26 is a flow diagram illustrating correction determination processB in a case where the correction information to be determined isfrequency correction information. As discussed above with reference toFIG. 23, this process S2312 a can be applied to any of the base stationsin the neighbourhood 2114, regardless of whether or not offsetinformation has been received by the communication manager 2118.

Correction determination process B starts with a step S2602 of accessingthe storage unit 2130 (for example, using the storage access circuitry2128) to obtain location information of the selected base station. Thestorage unit 2130 may be accessed using the identifier of the selectedbase station, for example.

In step S2604, location and velocity information (vehicle trackinginformation) of the moving vehicle (an aircraft, in this example) isobtained from an external database, such as an aviation database 2124.

Using the base station location information and the vehicle locationinformation, a vector separation r between the moving vehicle and theselected base station is determined in step S2606. This step may, forexample, be carried out by distance computation circuitry 2132.

In step S2608, the frequency offset 2Δf is calculated using the vectorseparation r and the velocity v of the vehicle. The offset 2Δf iscalculated according to the Doppler formula, as in the examplesdiscussed above:

${\Delta \; f^{FL}} = {\frac{r \cdot v}{{r}c}f_{c}^{FL}}$

where the offset 2Δf is double the value calculated from the aboveformula.

Finally, once the frequency correction information has been calculated,it is stored at step S2610 in the storage unit 2130.

FIG. 27 is a flow diagram illustrating correction determination processB in a case where the correction information to be determined is timingcorrection information. As discussed above with reference to FIG. 23,this process S2312 b can be applied to any of the base stations in theneighbourhood 2114, regardless of whether or not offset information hasbeen received by the communication manager 2118.

Correction determination process B starts with a step S2702 of accessingthe storage unit 2130 to obtain location information of the selectedbase station.

In step S2704, location information of the moving vehicle is obtainedfrom an external database, such as an aviation database 2124.

Using the base station location information and the vehicle locationinformation, a (scalar) separation distance r is determined in stepS2706. This step may, for example, be carried out by distancecomputation circuitry 2132.

In step S2708, the timing offset is calculated based on the separationdistance r, using for example the process discussed earlier withreference to FIG. 14.

Finally, once the timing correction information has been calculated, itis stored at step S2710 in the storage unit 2130.

As described above, FIGS. 24 to 27 all show examples of correctiondetermination processes A and B. As depicted in FIG. 23, once either ofthese processes has been completed, it is determined, in step S2314,whether the selected base station is the last base station. If so, themethod then proceeds to step S2316, and in response to a trigger, atleast a subset of the transmission adjustment control information storedin the storage unit 2130 is transmitted to the connected base station2104.

It should be noted that, while process A and process B have beendescribed separately, it is also possible for the two processes to becombined, in order to provide more accurate estimates of the timing orfrequency corrections.

According to the examples described above, a number of signals andmessages are passed between the components of the communication network2100. FIG. 28 is a timing diagram illustrating some of these messages.

In FIG. 28, messages passed between an air terminal 2102, a base station2104 that is currently in communication with the air terminal 2102 andan air-to-ground manager (ATGM) 2818 (an example of the communicationmanager 2118 depicted in FIG. 21) are illustrated.

The current base station (connected base station) 2104 receivescommunication signals (not shown) transmitted by the air terminal. Fromthese signals, the base station 2104 is able to calculate timing andfrequency offsets 2802 between expected values and measured values oftiming and frequency of received signals. Once these measurements havebeen made by the base station for one or more received signals, timingadjustment information 2806 may be transmitted to the air terminal 2102.

In addition, the timing and frequency measurements may be transmitted tothe ATGM 2818 as a measurement report 2804. The measurement report 2804may be sent to the ATGM 2818 in response to a request from the ATGM2818, or it may be sent periodically, at predetermined time intervals.Alternatively, the measurement report may be transmitted every time asignal is received by the base station 2104 from the air terminal 2102.It should be noted that the measurement report may only comprise offsetmeasurements for one of frequency and timing, or it may comprise offsetmeasurements for both.

Once it has received the measurement report 2804, the ATGM 2818 isconfigured to determine timing and/or frequency adjustment information2808 according to any of the techniques described above. The ATGM 2818is then configured—either automatically or in response to a trigger—totransmit adjustment information 2810 relating to other, non-connectedbase stations in the neighbourhood to the connected base station 2104.The connected base station 2104 may retain this information for its ownuse—for example, for use in determining a handover candidate—or it maypass on a portion of the adjustment information 2812 to other basestations in the neighbourhood. In particular, the current base station2104 may transmit adjustment information relating to a particular otherbase station to that base station. It should be noted that, althoughFIG. 28 shows the adjustment information for non-connected base stationsbeing transmitted to the connected base station 2104, it might insteadbe transmitted directly to the non-connected base stations from the ATGM2818.

The ATGM 2818 also transmits timing and adjustment information 2814 forthe connected base station 2104 to the base station 2104. Thisinformation is then transmitted by the base station 2104 to the airterminal 2102. This allows the air terminal 2102 to use the adjustmentinformation to adjust the frequency and/or timing of signals ittransmits to the connected base station 2104.

FIG. 28 depicts multiple signals transmitted within the communicationnetwork 2100. FIG. 29, meanwhile, depicts an example of the form thesemessages might take.

FIG. 29 depicts an IP (Internet Protocol) packet 2900. The IP packet2900 has a plurality of fields, including a base station ID field 2902for storing an identifier of a base station; in particular, the basestation identified in the base station ID field 2902 may be a basestation to which the packet 2900 is being transmitted, or from which itis being received.

The IP packet also comprises an AST ID field 2904, for storing anidentifier of an air terminal (or a vehicle terminal if the system isnot an ATG system). In particular, the air terminal identified in theAST ID field 2904 is the terminal for which the timing and/or Dopplercorrections (also included in the packet 2900) have been determined.That is, the air terminal identified is the ultimate destination for thepacket.

The packet 2900 includes a timing advance field 2906 for storing thecalculated timing adjustment information for the identified airterminal, and a Doppler offset field 2908, for storing the calculatedfrequency adjustment information for the identified air terminal.

Finally, the packet comprises a relative/absolute field 2910, forstoring a relative/absolute bit for identifying whether the timing andfrequency adjustment information is relative adjustment information orabsolute adjustment information.

The advantage of transmitting messages in the communication network 2100using an IP packet 2900 as illustrated in FIG. 29, is to ensurecompatibility of the mechanisms of the present technique with existingStandards. This ensures that the system can be more easily deployed andintegrated into existing systems.

In the examples described above, transmission adjustment controlinformation is determined for an air terminal, in relation to a numberof base stations in a neighbourhood of nearby base stations. Thetransmission adjustment control information includes frequencyadjustment information and/or timing adjustment information to beapplied by signals transmitted by the air terminal. The frequencyadjustment information indicates a frequency correction to be applied tothe transmitted signals in order to account for a frequency offset inthe signals caused by the Doppler effect. The timing adjustmentinformation indicates a timing correction to be applied to thetransmitted signals to reduce a timing offset in the signals caused bythe distance over which the signals need to travel. In particular, thetiming offset may relate to a connection setup (RACH) signal, which isexpected to be received in a particular sub-frame in a givencommunication frame.

In some of the above examples, the transmission adjustment controlinformation is computed centrally, in a communication manager on theground. This enables transmission adjustment control information forbase stations not currently in communication with the air terminal to becalculated, which allows a handover procedure transitioningcommunication from the current base station to another base station tobe carried out more smoothly. Performing the calculation centrally alsoallows the system to be implemented without requiring alterations to bemade to existing air terminals and base stations. This allows thepresent technique to be implemented such as to be compatible withexisting systems and current Standards, with minimal impact to existingusers.

While the present techniques have largely been described in terms of anair terminal installed in an aircraft, it will be appreciated that thisis just one example of a situation in which the present technique may beimplemented. The above described examples apply equally to acommunication system in any moving vehicle, for example a high speedtrain. However, it is also noted that the present technique isparticularly beneficial in air-to-ground systems, due to the high speedswith which aircraft travel, and the large distances between basestations on the ground.

In the present application, the words “configured to . . . ” are used tomean that an element of an apparatus has a configuration able to carryout the defined operation. In this context, a “configuration” means anarrangement or manner of interconnection of hardware or software. Forexample, the apparatus may have dedicated hardware which provides thedefined operation, or a processor or other processing device may beprogrammed to perform the function. “Configured to” does not imply thatthe apparatus element needs to be changed in any way in order to providethe defined operation.

Although particular embodiments have been described herein, it will beappreciated that the invention is not limited thereto and that manymodifications and additions thereto may be made within the scope of theinvention. For example, various combinations of the features of thefollowing dependent claims could be made with the features of theindependent claims without departing from the scope of the presentinvention.

1. An apparatus comprising: base station location identifying circuitryto obtain base station location information for a plurality of basestations that provide a wireless network for communication with a movingvehicle, the plurality of base stations comprising a current basestation connected with the moving vehicle and one or more other basestations; moving vehicle tracking circuitry to obtain moving vehicletracking information for the moving vehicle; correction determinationcircuitry to determine, based on the moving vehicle tracking informationand the base station location information, transmission adjustmentcontrol information associated with each other base station; and aninterface configured to transmit, for reception by the moving vehicle,the transmission adjustment control information associated with at leasta selected other base station, to enable the moving vehicle to adjust asignal transmitted to the selected other base station when a handoverprocedure is performed to transition communication with the movingvehicle from the current base station to the selected other basestation.
 2. The apparatus of claim 1, wherein the transmissionadjustment control information comprises at least one of: frequencyadjustment information indicative of a frequency adjustment to beapplied to a transmission frequency of said signal, so as to reduce afrequency difference between an observed frequency of the signal at theselected base station and a predetermined uplink frequency; and timingadjustment information indicative of a timing adjustment to be appliedto a transmission time of the signal, so as to reduce a timingdifference between a reception timing of the signal at the selected basestation and an expected timing.
 3. The apparatus of claim 2, wherein:the adjustment control information comprises absolute adjustment controlinformation or relative adjustment control information, wherein: theabsolute adjustment control information comprises at least one of anabsolute frequency adjustment and an absolute timing adjustment to beapplied to the signal as generated by a terminal device of the movingvehicle; and the relative adjustment control information comprises atleast one of a relative frequency adjustment and a relative timingadjustment to be applied to the signal in addition to at least one of anexisting frequency adjustment and an existing timing adjustment.
 4. Theapparatus of claim 1, wherein: the moving vehicle tracking informationcomprises information indicative of a location and a velocity of themoving vehicle.
 5. The apparatus of claim 4, wherein: the interface isconfigured to receive, from the current base station, identificationinformation of the moving vehicle; and the moving vehicle trackingcircuitry is configured to obtain the location and the velocity of themoving vehicle by accessing a tracking information database using theidentification information of the moving vehicle.
 6. The apparatus ofclaim 5, comprising distance computation circuitry configured todetermine, for each other base station, separation informationindicating a separation between the moving vehicle and that other basestation based on the location of the moving vehicle and a location ofthat other base station.
 7. The apparatus of claim 6, wherein: thetransmission adjustment control information comprises said frequencyadjustment information; the separation information identifies a vectorseparation; and the correction determination circuitry is configured todetermine the frequency adjustment information associated with eachother base station based on the vector separation between the movingvehicle and that other base station and the velocity of the movingvehicle.
 8. The apparatus of claim 6, wherein: the transmissionadjustment control information comprises said timing adjustmentinformation; and the correction determination circuitry is configured todetermine the timing adjustment information associated with each otherbase station based on the separation between the moving vehicle and thatother base station.
 9. The apparatus of claim 1, wherein: the basestation location identifying circuitry is configured to identify the oneor more other base stations with reference to a bearing of the movingvehicle.
 10. The apparatus of claim 1, wherein: the correctiondetermination circuitry is configured to perform a process ofdetermining further transmission adjustment control informationassociated with the current base station; and the interface isconfigured to transmit, for reception by the moving vehicle, the furthertransmission adjustment control information, to enable the movingvehicle to adjust at least one further signal transmitted to the currentbase station.
 11. The apparatus of claim 10, wherein: the correctiondetermination circuitry is configured to iteratively perform saidprocess, to enable ongoing adjustment of signals to be transmitted bythe moving vehicle to the current base station.
 12. The apparatus ofclaim 10, wherein: the interface is configured to receive offsetinformation for a plurality of previous signals received at the currentbase station from the moving vehicle, the offset information comprisingat least one of frequency offset information indicative of a differencebetween an observed frequency of each of the plurality of previoussignals received at the current base station and a predetermined uplinkfrequency of that previous signal, and timing offset informationindicative of a difference between a reception timing of each of theplurality of previous signals at the current base station and anexpected timing for that previous signal; and the correctiondetermination circuitry is configured to determine the furthertransmission adjustment control information based on the offsetinformation.
 13. The apparatus of claim 12, wherein the correctiondetermination circuitry is configured to determine the furthertransmission adjustment control information by calculating a filteredestimate from the offset information received for said plurality ofprevious signals.
 14. The apparatus of claim 6, wherein: the correctiondetermination circuitry is configured to perform a process ofdetermining further transmission adjustment control informationassociated with the current base station; the interface is configured totransmit, for reception by the moving vehicle, the further transmissionadjustment control information, to enable the moving vehicle to adjustat least one further signal transmitted to the current base station; thefurther transmission adjustment control information associated with thecurrent base station comprises frequency adjustment informationassociated with the current base station; the distance computationcircuitry is configured to determine, for the current base station, avector separation between the moving vehicle and the current basestation based on the location of the moving vehicle and a location ofthe current base station; and the correction determination circuitry isconfigured to determine the frequency adjustment information associatedwith the current base station based on the vector separation between themoving vehicle and the current base station and the velocity of themoving vehicle.
 15. The apparatus of claim 6, wherein: the correctiondetermination circuitry is configured to perform a process ofdetermining further transmission adjustment control informationassociated with the current base station; the interface is configured totransmit, for reception by the moving vehicle, the further transmissionadjustment control information, to enable the moving vehicle to adjustat least one further signal transmitted to the current base station; thefurther transmission adjustment control information associated with thecurrent base station comprises timing adjustment information associatedwith the current base station; the distance computation circuitry isconfigured to determine, for the current base station, furtherseparation information indicating a separation between the movingvehicle and the current base station based on the location of the movingvehicle and a location of the current base station; and the correctiondetermination circuitry is configured to determine the timing adjustmentinformation associated with the current base station based on theseparation between the moving vehicle and the current base station. 16.The apparatus of claim 1, wherein the interface is configured totransmit the transmission adjustment control information to the currentbase station, for reception by the moving vehicle.
 17. The apparatus ofclaim 16, wherein the transmission adjustment control information istransmitted in an IP packet comprising: identification information ofone of the plurality of base stations; a relative bit indicative ofwhether the transmission adjustment control information comprisesrelative adjustment control information or absolute adjustment controlinformation; the transmission adjustment control information; andidentification information of the moving vehicle.
 18. The apparatus ofclaim 1, wherein the interface is configured to transmit thetransmission adjustment control information associated with the selectedother base station for reception by said selected other base station.19. The apparatus of claim 18, wherein: the transmission adjustmentcontrol information associated with the selected other base stationcomprises timing adjustment information associated with the selectedother base station for enabling the selected other base station todetermine a reception timing of said signal transmitted to the selectedother base station when said handover procedure is performed totransition communication with the moving vehicle from the current basestation to the selected other base station.
 20. The apparatus of claim1, wherein the moving vehicle is an aircraft.
 21. A method comprising:obtaining base station location information for a plurality of basestations that provide a wireless network for communication with a movingvehicle, the plurality of base stations comprising a current basestation connected with the moving vehicle and one or more other basestations; obtaining moving vehicle tracking information for the movingvehicle; determining, based on the moving vehicle tracking informationand the base station location information, transmission adjustmentcontrol information associated with each other base station; andtransmitting, for reception by the moving vehicle, the transmissionadjustment control information associated with at least a selected otherbase station, to enable the moving vehicle to adjust a signaltransmitted to the selected other base station when a handover operationprocedure is performed to transition communication with the movingvehicle from the current base station to the selected other basestation.
 22. An apparatus comprising: means for obtaining base stationlocation information for a plurality of base stations that provide awireless network for communication with a moving vehicle, the pluralityof base stations comprising a current base station connected with themoving vehicle and one or more other base stations; means for obtainingmoving vehicle tracking information for the moving vehicle; means fordetermining, based on the moving vehicle tracking information and thebase station location information, transmission adjustment controlinformation associated with each other base station; and means fortransmitting, for reception by the moving vehicle, the transmissionadjustment control information associated with at least a selected otherbase station, to enable the moving vehicle to adjust a signaltransmitted to the selected other base station when a handover operationprocedure is performed to transition communication with the movingvehicle from the current base station to the selected other basestation.